Update Rubocop from 0.26.0 -> 0.26.1 for Travis builds

Change-Id: I8342d80e2de8dac5e088259ec6ef8f3a28e8343d
Reviewed-on: http://gerrit.causes.com/42990
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>