Use `_enum_methods_module` method instead of `@_enum_methods_module`
Use `_enum_methods_module` method instead of `@_enum_methods_module`

In Rails 4.2.6 and Rails 5.0.0.beta3, `ActiveRecord::Enum` provide
`_enum_methods_module` accessor method privately.
Sure `@_enum_methods_module` has been set by `enum` method
when we initialize `Event`, but I think it is better to use
accessor method to reduce dependency for `ActiveRecord::Enum`
internal implementation.

amatsuda/stateful_enum,amatsuda/stateful_enum