Enable RSpec/HookArgument cop and fix issues

To be consistent about before/after arguments, as `:each` or `:example`
 are same and default scopes, best not to send an argument in those
 scenarios.

Read about cop at http://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/HookArgument

spec/controllers/admin/api/stats_controller_spec.rb

@@ -16,7 +16,7 @@ describe Admin::Api::StatsController do
     end
 
     context 'events present' do
-      before :each do
+      before do
         time_1 = DateTime.parse("2015-01-01").in_time_zone
         time_2 = DateTime.parse("2015-01-02").in_time_zone
         time_3 = DateTime.parse("2015-01-03").in_time_zone