Exclude platform test vendor dir from rubocop.
Exclude platform test vendor dir from rubocop.

There are some ruby files in there that don't follow our rules.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf