Make Travis CI work

I could not get Travis Ci working. By copying some configuration file from the `.travis.yml` from the [Fataiando a Terra package](https://github.com/leouieda/fatiando/blob/master/.travis.yml) I could get it working.

But the tests are still not working. That happens because `S4` needs IDL to run and it is a proprietary software. When remove this dependency, I will the tests to run.

Related to #21
Closes #24