Update nodejs to v6 (dotnet#17)

- Default nodejs on Ubuntu 16.04 is v4.2.6, but SignalR needs v6 or later
- Symlink should no longer be required
- npm is now included in the nodejs v6 package, so there's no need to install a separate package

Author: mikeharder

BuildAgent/SetupScripts/BuildAgentSetup_Ubuntu.sh

@@ -39,11 +39,8 @@ echo "Installing Java..."
 apt-get install -y default-jre-headless unzip
 
 echo "Installing Node.js..."
+curl -sL https://deb.nodesource.com/setup_6.x | sudo -E bash -
 apt-get install -y nodejs
-ln -s /usr/bin/nodejs /usr/bin/node
-
-echo "Installing NPM..."
-apt-get install -y npm
 
 echo "Installing Bower globally..."
 npm install -g bower