Add requirements and bump versions

[AngelOnFira]

This closes #6 by adding Whoosh as a dependency of the package. It also bumps versions used in tox, removes explicitly stated Python 2.7 support and adds Python 3.7. 2.7 was removed as it is approaching EOL. The bumped versions of the testing suite aren't compatible with 2.7, so I thought it best to just remove support of it now.

Verified to work with Python 3.5 - 3.7 and Django 1.8 - 2.2. I can't install Python 3.4 with pyenv, so I'm only assuming it will work.

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ERROR:  py34-django18: InterpreterNotFound: python3.4
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ERROR:  py34-django22: InterpreterNotFound: python3.4
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