Make sure to ignore the /public/test-assets directory from Rubocop.

Since we're now precompiling the assets during tests, we need to
exclude this directory from Rubocop so it doesn't pick up files in there
on subsequent runs.

.rubocop.yml

@@ -4,6 +4,7 @@ AllCops:
     - vendor/**/*
     - config/compass.rb
     - public/web-assets/**/*
+    - public/test-assets/**/*
   RunRailsCops: true
 
 Lint/LiteralInInterpolation: