enable Travis CI

[bplimley]

Closes #5

[bplimley]

Travis CI is free for public repos. Private repos are allowed a 100-build trial period.

Right now I'm only running for Python 2.7 and 3.6, and only on master branch. I'm not sure if each version counts as a separate build or not. Regardless, we can do more once we go public.

[bplimley]

Also, it failed and I don't have time to figure it out right now.

[bplimley]

travis-ci/travis-ci#2650 ...

[bplimley]

Yay, Travis says our code works!

I put in a try/except to bypass the spectrum file plotting, which throws a tkinter.TclError error on Travis because there is no $DISPLAY. At some point we should revise those tests to check the spectra without requiring a visual check, and if we still want plotting in the tests then we should find a more elegant workaround. (I thought about importing tkinter and except tkinter.TclError but in python 2 the package is Tkinter and in python 3 it's tkinter and I decided it wasn't worth going through the import complications...)

[markbandstra]

Looks good.