[WIP] Add coverage to travis tests

[bannanc]

I think this is all we have to do to generate coverage reports for tests, if it works here I can do it on openforcefield also.

[davidlmobley]

The README.md shows the code coverage badge, but the status is unknown. Is there a way to tell whether it's working before merging?

(Often I've seen code coverage tests/reports as part of a PR, but turning that on -- likely a toggle on GitHub's settings -- perhaps is a separate test. And maybe the badge only reports on coverage of master?)

Your branch is also out-of-date.

[bannanc]

Yes, I meant to put a [WIP] on this when I realized there was something up with the tests. The only coverage I've done was with pytest and directly to a master branch so I'm not sure if the report will show up before this is merged into the master.

[bannanc]

For the record this is trying to address #262 and #264

[bannanc]

I've been revisiting my github PRs and Issues and found this. I think it would be good for our (mine and @camizanette 's) portfolios for this to not be failing. I think the current issues here relate to the openeye license being expired and maybe a version problem with networkx. I'm going to try to get things passing and then maybe pin versions so we know it is reproducible within the environment it was developed in.

[bannanc]

Note on that, going to doesn't mean immediately, just means at some point.

[davidlmobley]

All for pinning versions; having the tests frequently start failing because of a change in somethign we don't control is frustrating.