Added travis config, fixed tests for use on travis

I've added travis configuration and updated the tests to work with travis.

@ljharb If you can enable the hook, then this PR should already show that it's working.

Current travis build: https://travis-ci.org/koenpunt/nvm/builds/20936607

~~Although there is still an issue with urchin; it always exits with zero, so a failed build will not show as such, but [I've filed an issue](https://github.com/scraperwiki/urchin/issues/5) with urchin for this. I think we can easily resolve this issue ourselves as well.~~ 
**The issue has been resolved and urchin exits with non-zero status on failures.**