Pin rubocop to 0.30.0
Pin rubocop to 0.30.0

The most recent sweetness build just picked up a bug in the Performance/Sample
cop (https://github.com/bbatsov/rubocop/issues/1816) from 0.30.1, which fails
the sweetness build. Let's pin rubocop to a known good version so that we're
not continually picking up new cops and changes from patch versions without
explicit updates to aptible-tasks.

aptible/aptible-tasks