Pin Rubocop version to 0.20.1

The most recent version of Rubocop adds some lints which results in our
build failing.

Since we can't predict when Rubocop will be updated, it is best to pin
the version so we can control whether or not the build breaks.

Change-Id: I585af6f01ab2d344e3802fea762c0e812e4246e8
Reviewed-on: http://gerrit.causes.com/37557
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>