Lock Rubocop down at `~> 0.17.0`

As much as I like playing whack-a-mole with style errors, this has to
stop. The cops are varying wildly from minor version to minor version.
This makes accepting PRs from the community very hard. We are locking
to `0.17.0` as this was the last version that wasn't total BS.
Merge pull request #146 from opscode/schisamo/lock-rubocop-down

Lock Rubocop down at `~> 0.17.0`