Upgrade rubocop and rubocop-rspec gems to their next major version

Context

Some people need to have the latest version of Rubocop available:

• #34 (closed)
• #35 (closed)
• gitlab-org/gitlab#356738 (closed)

Goal

• Have rubocop and rubocop-rspec gems at their next major versions
• All specs are passing

Hints

• After having spent a bit of time on it, the upgrade is not trivial, and we have to do both gems at the same time...
• rubocop 1.x upgrade guide
• rubocop-rspec 2.x upgrade guide