[WIP] Travis CI yml

I'm submitting this pull request for a `.travis.yml` I've been constructing for OpenMM. The good news is that the tests I have Travis doing seem to complete in a reasonable amount of time. However, I don't think I'm installing the python api correctly (and thus am not correctly testing it)...if anyone has any ideas here let me know. Also, if there are tests that should not be done, or tests that should be done but I am not including, let me know and I'll change the yml accordingly. I've marked it [WIP] because I expect that I may need to make some changes, but wanted to let everyone know that I'm in the process of making it.

Hope this helps.