[QA] Rubocop style checker

[Matthew Flaschen]

It looks like there is a new Ruby style checker called Rubocop.  It is 
running as a non-voting job on Flow (and I assume other repositories).

If this is considered ready to run, please document this at 
https://www.mediawiki.org/wiki/Manual:Coding_conventions/Ruby .

Otherwise, people see mysterious FAILURE messages on all their 
changesets without knowing what style guide or tool they're supposed to 
work from.

It's also currently failing completely 
(https://integration.wikimedia.org/ci/job/mwext-Flow-bundle-rubocop/33/console):

18:13:53 bundler: command not found: rubocop
18:13:53 Install missing gem executables with `bundle install`

It seem like there is an issue with the configuration.

Matt Flaschen