Add HTML Proofer for CI

This pull requests adds a single `.travis.yml` which implements continuous integration checking via Travis CI. Specifically, on each pull request, the following will be checked:

* The site actually builds (e.g., no Jekyll syntax errors)
* All links are valid, including existence of in-site anchors
* All images are valid and have proper alt tags
* All scripts are valid

If all of the above passes, you'll see a big :green_heart: status icon on the pull request, letting you know you can merge with confidence ([example of a successful build](https://travis-ci.org/benbalter/codingforlawyers/builds/35435974)). If not, you'll see a :x:, indicating there's an error ([example of an erred build for this site](https://travis-ci.org/benbalter/codingforlawyers/builds/35435021)).

To note, in running it, it found two bad links, including a http/https swap, and a improper internal anchor.

@konklone, I know you were sensitive about not bloating the root with system files, but I think a single `.travis.yml` file (rather than e.g., a Gemfile + scripts) strikes a good balance as the feedback provided will be helpful for new users.

@vzvenyach If merged, you'll need to tell Travis to watch your Repo, which you can do in ~30 seconds by:

1. Go to https://travis-ci.org/profile
2. Oauth against GitHub
3. Click the On button next to this repo in the list

(note, the CI service is free and requires no maintenance once initially configured)