Use mock modules to build docs on RTD faster

Read the Docs was taking lots of time and randomly crashing all the time. That might be because it depended on modules with C extensions, which [aren't supported on RTD](http://read-the-docs.readthedocs.org/en/latest/faq.html#i-get-import-errors-on-libraries-that-depend-on-c-modules) for security reasons. This should fix that.