Upgrade Rubocop to 0.15.0

In the process, call `inspect_file` via `send` since it is now private.

Change-Id: I8b69e1d3a4d6c31a53a08325085115326a2fe60a
Reviewed-on: https://gerrit.causes.com/30956
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>