Pin Rubocop in Gemfile instead of gemspec

We don't want to declare this as a development dependency because it's
not really a dependency--it's simply a tool we run against the
repository as part of our CI runs.

Change-Id: Ica1bdbe5de39ee88f8e114a7cbce4cc6e3937844
Reviewed-on: http://gerrit.causes.com/45031
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Upgrade Rubocop 0.26.0 -> 0.27.1 for Travis builds

Change-Id: I2886af1b3fef4d4445db1c9b8f4b781a555272b3
Reviewed-on: http://gerrit.causes.com/45032
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>