ENH: Better tests

Ready for review/merge.

Closes #179.
@rkmaddox I added this line just so we can be certain :)

But FYI this line activates the 3.3 test env (from `master`):

https://github.com/LABSN/expyfun/blob/master/.travis.yml#L31

I also just committed a change that will make it so we use 3.4 instead of 3.3 for maximum future compatibility, hopefully that doesn't break anything.