Add travis configuration

see https://travis-ci.org/dnozay/lets-encrypt-preview/builds/42272720
you will need to setup an account on travis-ci and enable it there. Here is a [shameless plug](http://dnozay.github.io/testing/2014/10/25/testing-github-project.html) if you don't know how to.
btw, wouldn't this line require to `sudo apt-get install python-pip` in `before_install`?