Add code coverage

[bannanc]

We learned how to do code coverage at MolSSI this week, it is really simple to set up assuming you're running tests with pytest. You just add a line to your travis.yml and go to codecov.io to add the repository and get the link for the badge.

I can look into the ways to do this with nosetests instead or convert our tests over to pytest.

[davidlmobley]

I think converting to pytest and getting code coverage going makes a lot of sense, though is more important onopenforcefield than here.

[bannanc]

I think we have an issue on it for openforcefield too. I feel more comfortable using this one for testing the set up since there isn't active development happening.