Mention the possibility to upgrade the play installation on version mismatch.

I just updated my project to play 2.2.1 and started it with my old 2.2.0 installation.
The error message

    This project uses Play 2.2.1!
    Update the Play sbt-plugin version to 2.2.0 (usually in project/plugins.sbt)

suggested a thing I wasn't interested in, instead it should have told me to update
my play installation. Both suggestions make sense.

One could even compare versions to give a more educated suggestion (detect upgrade/
downgrade), but because this should also support RCs and clever but still still short
suggestions/messages it's not done in a second and it's probably not worth the effort.
Merge pull request #1964 from magro/fix-play-version-mismatch-msg

Mention the possibility to upgrade the play installation on version mismatch