gh-113: update C API example

* update download instructions for CentOS 6
* remove sudo from RedHat repositories, they prefer 'su' and
  docker images even don't have 'sudo'.
  Recommend using root command prompt instead.

Author: kostja

doc/book/app_server.rst

@@ -142,7 +142,7 @@ The C file should look like this:
     #include <lua.h>
     #include <lauxlib.h>
     #include <lualib.h>
-    #include <tarantool.h>
+    #include <tarantool/module.h>
     static int
     myfun(lua_State *L)
     {

download.rst

@@ -161,38 +161,40 @@ Tarantool - Downloads
           | ``$basearch`` (i.e. base architecture) must be either i386
             or x86_64.
 
-          Copy and paste the script below to the terminal prompt:
+          Copy and paste the script below to the *root* terminal prompt:
 
           .. code-block:: bash
 
+              # Clean up yum cache
+              yum clean all
               # Enable EPEL repository
-              sudo yum -y install http://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
+              yum -y install http://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
               sed 's/enabled=.*/enabled=1/g' -i /etc/yum.repos.d/epel.repo
 
               # Add Tarantool repository
-              sudo rm -f /etc/yum.repos.d/*tarantool*.repo
-              sudo tee /etc/yum.repos.d/tarantool_1_7.repo <<- EOF
+              rm -f /etc/yum.repos.d/*tarantool*.repo
+              tee /etc/yum.repos.d/tarantool_1_7.repo <<- EOF
               [tarantool_1_7]
               name=EnterpriseLinux-\$releasever - Tarantool
-              baseurl=http://download.tarantool.org/tarantool/1.7/el/7/\$basearch/
+              baseurl=http://download.tarantool.org/tarantool/1.7/el/6/\$basearch/
               gpgkey=http://download.tarantool.org/tarantool/1.7/gpgkey
               repo_gpgcheck=1
               gpgcheck=0
               enabled=1
 
               [tarantool_1_7-source]
               name=EnterpriseLinux-\$releasever - Tarantool Sources
-              baseurl=http://download.tarantool.org/tarantool/1.7/el/7/SRPMS
+              baseurl=http://download.tarantool.org/tarantool/1.7/el/6/SRPMS
               gpgkey=http://download.tarantool.org/tarantool/1.7/gpgkey
               repo_gpgcheck=1
               gpgcheck=0
               EOF
 
               # Update metadata
-              sudo yum makecache -y --disablerepo='*' --enablerepo='tarantool_1_7' --enablerepo='epel'
+              yum makecache -y --disablerepo='*' --enablerepo='tarantool_1_7' --enablerepo='epel'
 
               # Install tarantool
-              sudo yum -y install tarantool
+              yum -y install tarantool
 
         * RHEL 7 and CentOS 7
 
@@ -203,13 +205,15 @@ Tarantool - Downloads
           | ``$releasever`` (i.e. CentOS release version) must be 7, and
           | ``$basearch`` (i.e. base architecture) must be either i386 or x86_64.
 
-          Copy and paste the script below to the terminal prompt:
+          Copy and paste the script below to the *root* terminal prompt:
 
           .. code-block:: bash
 
+              # Clean up yum cache
+              yum clean all
               # Add Tarantool repository
-              sudo rm -f /etc/yum.repos.d/*tarantool*.repo
-              sudo tee /etc/yum.repos.d/tarantool_1_7.repo <<- EOF
+              rm -f /etc/yum.repos.d/*tarantool*.repo
+              tee /etc/yum.repos.d/tarantool_1_7.repo <<- EOF
               [tarantool_1_7]
               name=EnterpriseLinux-\$releasever - Tarantool
               baseurl=http://download.tarantool.org/tarantool/1.7/el/7/\$basearch/
@@ -227,10 +231,10 @@ Tarantool - Downloads
               EOF
 
               # Update metadata
-              sudo yum makecache -y --disablerepo='*' --enablerepo='tarantool_1_7'
+              yum makecache -y --disablerepo='*' --enablerepo='tarantool_1_7'
 
               # Install Tarantool
-              sudo yum -y install tarantool
+              yum -y install tarantool
 
         * Amazon Linux
 
@@ -243,17 +247,19 @@ Tarantool - Downloads
           | ``$releasever`` (i.e. RHEL / CentOS release version) must be 6, and
           | ``$basearch`` (i.e. base architecture) must be either i386 or x86_64.
 
-          Copy and paste the script below to the terminal prompt:
+          Copy and paste the script below to the *root* terminal prompt:
 
           .. code-block:: bash
 
+              # Clean up yum cache
+              yum clean all
               # Enable EPEL repository
-              sudo yum -y install http://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
+              yum -y install http://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
               sed 's/enabled=.*/enabled=1/g' -i /etc/yum.repos.d/epel.repo
 
               # Add Tarantool repository
-              sudo rm -f /etc/yum.repos.d/*tarantool*.repo
-              sudo tee /etc/yum.repos.d/tarantool_1_7.repo <<- EOF
+              rm -f /etc/yum.repos.d/*tarantool*.repo
+              tee /etc/yum.repos.d/tarantool_1_7.repo <<- EOF
               [tarantool_1_7]
               name=EnterpriseLinux-\$releasever - Tarantool
               baseurl=http://download.tarantool.org/tarantool/1.7/el/6/\$basearch/
@@ -271,10 +277,10 @@ Tarantool - Downloads
               EOF
 
               # Update metadata
-              sudo yum makecache -y --disablerepo='*' --enablerepo='tarantool_1_7' --enablerepo='epel'
+              yum makecache -y --disablerepo='*' --enablerepo='tarantool_1_7' --enablerepo='epel'
 
               # Install Tarantool
-              sudo yum -y install tarantool
+              yum -y install tarantool
 
         * Snappy package