Rails 4.2.0-rc1 support

Fixes: https://github.com/byroot/activerecord-typedstore/issues/20

Things left to do:
  - [ ] Workaround the warning
  - [ ] Blocked by a bug upstream: https://github.com/rails/rails/issues/17900

Otherwise everything is green.




FYI this is going to break for sure in 5.0. Since this is going to hack on internals anyway, might I suggest hooking into `attribute` which is less likely to change in the future and will probably become public API in 5.0?

```ruby
attribute :foo, Type::Whatever.new
```

https://github.com/rails/rails/blob/master/activerecord/lib/active_record/attributes.rb