Add basic Rubocop configuration

Copied from the Overcommit repository.

Change-Id: I9f3129e0d2cd95d2d7cbb4fdc9b67af5e8d4ff73
Reviewed-on: http://gerrit.causes.com/36135
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>