fix dependency on rails/test_unit/sub_test_task

Merge https://github.com/audionerd/draper/tree/rails5 to fix dependency
on rails/test_unit/sub_test_task

Fixes drapergem#681

lib/draper/tasks/test.rake

@@ -1,6 +1,6 @@
 require 'rake/testtask'
 
-test_task = if Rails.version.to_f < 3.2
+test_task = if Rails.version.to_f < 3.2 || Rails.version.start_with?('5')
   require 'rails/test_unit/railtie'
   Rake::TestTask
 else