rubocop: ignore style warnings for db schema

This suggests dumb things like treating date numbers like 2012_07_04
to be split at every third character. And it fights with the rails
autogeneration, which puts a blank line at the beginning of the block
(but rubocop really wants to remove it, so you go back and forth as
rails and rubocop are run).

.rubocop.yml

@@ -11,7 +11,10 @@ AllCops:
 Performance:
   enabled: true
 
-Style/FrozenStringLiteralComment:
+Style:
+  Exclude:
+    - db/schema.rb
+Layout:
   Exclude:
     - db/schema.rb