Enable travis runs for pull requests.

[thomasdziedzic]

I opened #10 and I expected travis to run automatically. It did not.

[MartijnSch]

It would be good to have this, did your PR fix this?

[thomasdziedzic]

No, I think you have to setup the travis integration as the project owner.

Check out https://docs.travis-ci.com/user/getting-started/