Update play.rb: show caveat for new version

In Play 2.3 the play command has become the activator command. Play
has been updated to use Activator, so people should use
typesave-activator instead which already has a formula.

You can read more about this change at:
http://www.playframework.com/documentation/2.3.x/Migration23
and
http://www.playframework.com/documentation/2.3.x/Highlights23
mend