Make rubocop ignore remote repositories

The introduction of rubocop (Commit 70f3919) missed blacklisting files
which are not part of mediawiki/core.  Developers running rubocop
against their working copy would end up with errors reported against
code which is out of the mediawiki/core repository.

Instruct rubocop to ignore some well known paths:

Place holders for additional repositories:
- extensions
- skins
- vendor

And tests/frontend/node_modules which is populated via npm install.

Change-Id: I1abdaa3ef110d981e3c017667f87e698016d6805