Add Travis tests for site

[doktorbro]

This should prevent the documentation from nasty 404s.

[doktorbro]

My evil plan doesn’t work.

[parkr]

This has increased the build time significantly. Installing html-proofer and its dependencies took a very long time, even though the rake task failed. Any way to speed this up? If all builds (even builds without docs changes) have to run all of these, I'd rather skip them.

[doktorbro]

Installing html-proofer and its dependencies took a very long time.

This is true. I thought Jekyll’s and Proofer’s dependencies intersect. But Proofer alone take up to 5 minutes for installation.