Fix rubocop config

`undefined method '[]' for nil:NilClass`

Author: bf4

Rakefile

@@ -29,7 +29,7 @@ else
     Rake::Task[:rubocop].clear if Rake::Task.task_defined?(:rubocop)
     desc 'Execute rubocop'
     RuboCop::RakeTask.new(:rubocop) do |task|
-      task.options = ['--rails', '--display-cop-names', '--display-style-guide']
+      task.options = ['--display-cop-names', '--display-style-guide']
       task.fail_on_error = true
     end
   end