Update Rubocop to ruby 2.3

.rubocop.yml

app/controllers/projects/wikis_controller.rb

app/mailers/notify.rb

app/models/application_setting.rb

app/models/concerns/issuable.rb

app/models/concerns/milestoneish.rb

app/models/group_milestone.rb

app/models/project.rb

app/models/user.rb

app/models/wiki_page.rb

app/services/create_tag_service.rb

app/services/delete_tag_service.rb

app/services/projects/create_service.rb

app/workers/authorized_projects_worker.rb

lib/api/branches.rb

lib/api/entities.rb

lib/gitlab/ci/config/entry/configurable.rb

lib/gitlab/email/message/repository_push.rb

lib/gitlab/git/blob_snippet.rb

lib/gitlab/metrics.rb