Use truncate_all instead of DatabaseCleaner

Performance tests show both methods of truncating the database take
about the same time, so we can remove one dependency and we don't lose
anything in the process.

spec/spec_helper.rb

@@ -45,7 +45,7 @@ RSpec.configure do |config|
     example.run
     self.use_transactional_tests = true
 
-    DatabaseCleaner.clean_with(:truncation)
+    ActiveRecord::Tasks::DatabaseTasks.truncate_all
     Rails.application.load_seed
   end