Change Rubocop flags to be 0.12.0-compatible

As of 0.12.0, the `rubocop` CLI no longer requires the `--silent` flag,
and will output a warning if it is used.

Remove the flag from the Ruby style pre-commit check. Note that this
makes this no longer backwards compatible with older versions of
Rubocop, but the 'Unexpected output from rubocop' warning message will
hopefully direct people to upgrade.

Change-Id: Iec239636e367ddfda5fda05920e79981f27ea076
Reviewed-on: https://gerrit.causes.com/28337
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Ryan Fitzgerald <ryan.fitzgerald@causes.com>
Cut version 0.2.5

Change-Id: I5da3d42ab73dc75242944f3e0941c4e4b79ed983
Reviewed-on: https://gerrit.causes.com/28338
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Ryan Fitzgerald <ryan.fitzgerald@causes.com>