Update play formula: add caveat

In Play 2.3 the play command has become the activator command. Play has
been updated to use Activator.

People should use typesafe-activator formula instead.

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