Disable new Rubo-Cops

We have enough violations already. Let's add them as we see fit and
avoid the warnings for now.

.rubocop_styleguide.yml

@@ -5,6 +5,7 @@
 # rubocop locally, the default configuration file `.rubocop.yml` loads
 # our "todo lists" to ignore all current violations.
 AllCops:
+  NewCops: disable
   TargetRailsVersion: 5.0
   Exclude:
     - 'bin/**/*'