PCC: add rubocop

[ndushay]

• add rubocop gem with reasonable rubocop.yml defaults to project;
• add rubocop rake task
• add rubocop run to build used by travis

consider pegging the version of rubocop so we aren't churning on code because some folks decided they like certain syntax better than other syntax.

from https://github.com/bbatsov/rubocop

RuboCop's development is moving at a very rapid pace and there are often backward-incompatible changes between minor releases (since we haven't reached version 1.0 yet). To prevent an unwanted RuboCop update you might want to use a conservative version locking in your Gemfile:

gem 'rubocop', '~> 0.49.1', require: false