Add Travis CI?

Hey @csswizardry,

I wondered if you'd consider adding Travis CI to this repository so that any Pull Requests to this project could be automatically checked:
- for syntax errors;
- and `$use-silent-classes` continues to suppress all output.

If adding Travis is something that you'd rather not have, feel free to reject this (I do feel a bit guilty about suggesting to add 3 files to a project that only has 4). I was actually using this project as a convenient example of `<blatantspam>`[testing an 'silent-all-classes' variable could be verified automatically](http://mattandre.ws/2014/01/testing-sass-travis-ci/)`</blatantspam>` so it wouldn't be time wasted if the answer is no :-).

To benefit from this before merging this Pull Request you'll need to [switch Travis CI on](http://docs.travis-ci.com/user/getting-started/) for this repository :-)

PS. My text editor took the liberty of trimming the whitespace on your `README.md` file: I can undo this if you'd prefer.