When I updated IDEA from 14.1.4 to the 14.1.5 EAP (IU-141.2311) I saw a confusing message, screen shot attached. It said IDEA was outdated to use with the Scala plugin, followed by a mess of IDEA version numbers. I had my Scala update set to nightly.
After an uninstal and reinstall of the Scala plugin, all was well.
So I think that the message was mistaken, but in any case it is misformed.