Add RuboCop Ruby style pre-commit check

RuboCop (https://github.com/bbatsov/rubocop) is a Ruby code style
linter that performs a large collection of checks against a
collaborative community style guide
(https://github.com/bbatsov/ruby-style-guide). It will soon also have
support for Rails-specific lints (right now only alpha support for Rails
lints are available, so we aren't going to use it yet).

Add it as a default check for Ruby files.

Change-Id: I5717fa01a3d67d1f93efdd46b40ae8eae57eb04e
Reviewed-on: https://gerrit.causes.com/24663
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>