Trying to get Travis-CI builds working

This is where we get Travis-CI continuous integration working!

For some reason Github think my rwest/RMG-Py is no longer the same project as GreenGroup/RMG-Py and I cannot send pull requests. While I wait for them to fix this, I have pushed this topic branch to GreenGroup and am sending the pull request from there.
I wanted to do it via a pull request to see how the Travis-CI thing handles it, and because it may take several commits to get right…
Let's not merge until it seems to be working 
https://travis-ci.org/GreenGroup/RMG-Py/

Current status: [![Build Status](https://travis-ci.org/GreenGroup/RMG-Py.svg?branch=travis)](https://travis-ci.org/GreenGroup/RMG-Py)