Condense Ruby style output

The `rubocop` executable was outputting a lot of unnecessary text. Add
the `--format=emacs` and `--silent` flags to make the output more
concise.

Change-Id: Ib393c856e70327c4d670e9d57b19f39fc9b01f4e
Reviewed-on: https://gerrit.causes.com/25163
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>