Add Tests and Travis CI configuration

This PR adds tests that verify that the site compiles and there are no broken links. It configures Travis CI to run these tests automatically for all branches.Since html-proofer was complaining about a broken anchor link, I fixed that.
I'm curious why there was already an ignore for sass_cache, but I had a directory with a hyphen instead of an underscore. My sass version is 3.4.5.