Upgrade Rubocop to 0.25.0 for Travis builds

Change-Id: I556b5ad4c9d92f2d1c21f04648930a01b353a942
Reviewed-on: http://gerrit.causes.com/42207
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>