add test for long_description

[andamian]

publishing to pypi will fail if the long_description of the package is not formatted correctly. Packages in the repo use README.rst for long description. The fix for the problem is to add a test in Travis to fail on invalid package documentation.

New test should do something like:
python setup.py sdist; twine check dist/* and fail for errors. twine would need to be added to the dev_requirements.txt