Fix primary key issue for Rails 3.x and 4.0 and mysql 5.7

MySQL 5.7 no longer supports null default values for the primary key.
Override the Native default for primary keys in MySql.
https://stackoverflow.com/a/34555109

spec/helper.rb

@@ -38,6 +38,17 @@
 ActiveRecord::Base.logger = Delayed::Worker.logger
 ActiveRecord::Migration.verbose = false
 
+# MySQL 5.7 no longer supports null default values for the primary key
+# Override the default primary key type in Rails <= 4.0
+# https://stackoverflow.com/a/34555109
+if db_adapter == "mysql2" &&
+   (::ActiveRecord::VERSION::MAJOR == 3 ||
+   (::ActiveRecord::VERSION::MAJOR == 4 && ActiveRecord::VERSION::MINOR == 0))
+  class ActiveRecord::ConnectionAdapters::Mysql2Adapter
+    NATIVE_DATABASE_TYPES[:primary_key] = "int(11) auto_increment PRIMARY KEY"
+  end
+end
+
 migration_template = File.open("lib/generators/delayed_job/templates/migration.rb")
 
 # need to eval the template with the migration_version intact