Added RuboCop linter

Test Plan:
Add the following JSON to your .arclint:

"rubocop": {
  "type": "rubocop",
  "include": "(\\.rb$)"
}

Then, add a ruby file with errors, for instance:

def hello()
  puts 'world'
end

Run arc lint. It should come up with something along the line of: "Omit the parentheses in defs when the method doesn't accept any arguments."

Reviewers: joshuaspence, remon, Blessed Reviewers, epriestley

Reviewed By: joshuaspence, Blessed Reviewers, epriestley

Subscribers: reu, calfzhou, jjooss, cburroughs, chad, Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10738