Add build and links test

This adds a continuous testing with [Proofer](https://github.com/gjtorikian/html-proofer). 

> Proofer is a set of tests to validate your HTML output. These tests check if your image references are legitimate, if they have alt tags, if your internal links are working, and so on.

You must enable Travis to see it working.

Later you can expand the test suite by adding the JS linter.
The `vendor` directory is where Bundler install the gems.