Ask for rescan upgrade

https://bugs.launchpad.net/mixxx/+bug/1390019

Tell the user that we have to rescan the library for coverart when they are upgrading from 1.11.

It would be nice if a native speaker could look over the text that is displayed in the QMessageBox in upgrade.cpp
I don't like this style of comment, it is very LOUD.  Please just replace with 

// Upgrade to multi-library folder settings

(also there was a spelling mistake)