Removing unnecessary dependencies from requirements file (#416)

`boto3`: Many users might have this library already available in thier
Python installation from installing the AWS CLI. We use boto3 in
only one place, which is to get the region_name from `boto3.session.Session`
which hasn't changed since 2016. Updating the requirement to depend on
boto3 >=1.5.0 allows us to be compatible with existing installations that
might already have an older version of boto3

`botocore, docutils, futures, jmespath, python-dateutil, s3transfer`: These
packages are not used in the SAM translator but come as a dependency through
boto3. Hence removing the direct dependency.

`functools32`: Grepped for every single method offered by this package. We don't
use any of it directly within SAM Translator. This might have come as a
dependency through boto3.

`PyYAML`: Only required by the tests. So moving to dev dependency

`six`: Reducing the specificity of version requirement

`~=`: Using this operator in order to accept to a compatible minor/patch version
but preventing a major version change.

**Testing**:
- `make pr` and `make test` pass. Built a tar.gz distribution using
  `python setup.py sdist` and installed it in a clean virtualenv
  successfully. Tests ran successfully too. I was also able to import

Author: sanathkr

requirements/base.txt

@@ -1,12 +1,5 @@
-boto3>=1.6.2
-botocore>=1.9.2
-docutils==0.14
-enum34==1.1.6
-functools32==3.2.3.post2
-futures==3.2.0
-jmespath==0.9.3
-jsonschema==2.6.0
-python-dateutil==2.6.1
-PyYAML==3.12
-s3transfer==0.1.13
-six==1.11.0
+boto3~=1.5.0
+enum34~=1.1.6
+jsonschema~=2.6.0
+six~=1.11.0
+

requirements/dev.txt

@@ -3,6 +3,7 @@ flake8>=3.3.0
 tox>=2.2.1
 pytest-cov>=2.4.0
 pylint>=1.7.2
+PyYAML==3.12
 
 # Test requirements
 pytest>=3.0.7