Upgrade play-plugins-mailer

play-plugins-mailer 2.3.1 was released just now. It has some major improvements like finally got attachment support, debugging can be enabled and some nasty bugs got fixed. [Here](https://github.com/typesafehub/play-plugins/compare/mailer-2.3.0...mailer-2.3.1) is the diff.
@joscha Do you think it makes sense to also implement the attachment functionality into `play-easymail` or should people use the "base" library for it instead?