Automate tests using Travis

guss-rem is still used in a few places (the `rem()` function itself, not the mixin), and since it already has a test suite you might want to have a way to warn you if they break (if, for example, there is a new version of Sass, or you switch to libsass one day).

So here's a full Travis CI integration.

nota bene: if this PR is merged, you'll have to change the URL to the status image in the README and activate tracking of this repo in Travis.

![ ](http://tclhost.com/vdpVgV5.gif)