Adds Travis-CI testing

This commit adds [Travis-CI testing](https://travis-ci.org/), which allows
free and easy continuous integration for pull requests
and new commits. It also adds a `make deps` target
in the Makefile for easily adding the dependencies.

It can be enabled [here](http://docs.travis-ci.com/user/getting-started/#Step-one%3A-Sign-in).
Just follow instructions 1-2, three is already done. Thanks.