(PDOC-8) Add .rubocop.yml to project

Since we want to use rubocop to help maintain the code quality of
this project, add a .rubocop.yml file (borrowed from the puppet repo)
so that we can successfully enforce the things we want and disable
those we don't.
(PDOC-8) Fix present rubocop offenses

Since we want to run rubocop in travis when pull requests are
submitted, fix a few pre-existing rubocop offenses.
(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.
Merge remote-tracking branch 'upstream/pr/18'

* upstream/pr/18:
  (PDOC-8) Add rubocop to travis job
  (PDOC-8) Fix present rubocop offenses
  (PDOC-8) Add .rubocop.yml to project