Add Rubocop checks to Travis CI run

Ensure that future changes do not add new offences.

For this all existing offences need to be disabled for now, and have to
be solved step by step.

Change-Id: Ie9b3c557a1d5ce3a47e16ad71686ce7567b02650
Reviewed-on: http://gerrit.causes.com/37453
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>