(PDOC-8) Add rubocop to travis job

In order to ensure long term code quality, add a rubocop check to
the travis job which runs when a pull request is submitted. Do this
by adding a rake task for rubocop and setting up travis to run
both the spec rake task and the rubocop one.

Additionally, make a few changes to the .rubocop.yml file so it
will stop complaining about incorrect namespaces.