Add ActiveRecord::Migration.maintain_test_schema! to spec_helper

New in Rails 4.1, this eliminates spec failures due to forgetting to run
`db:test:prepare`.

spec/spec_helper.rb

@@ -44,3 +44,5 @@ RSpec.configure do |config|
     TestEnv.init
   end
 end
+
+ActiveRecord::Migration.maintain_test_schema!