Use docker (bug 1075456)

Fixes [bug 1075456](https://bugzilla.mozilla.org/show_bug.cgi?id=1075456)
To be honest I would personally simply add the docs requirements to the dev requirements. This would only make it install a few added dependencies in Travis, which isn't a big deal.

Also, it would allow us to have Travis test the doc build, which would be nice.

If that sounds reasonable, I'll create a bug for that.