From Yosemite to Mavericks
Restart your MacMini to recovery mode by pressing cmdR at startup.  

Open Utilities/Terminal in the menubar  



Please replace "YourMainVolume" with the name of your main Mac volume (probably "Macintosh HD") in the following commands.

If you made a backup of your SystemVersion.plist (like advised in my answer) enter:  

rm "/Volumes/YourMainVolume/System/Library/CoreServices/SystemVersion.plist"


and hit ↵, then enter:

mv "/Volumes/YourMainVolume/System/Library/CoreServices/SystemVersion.plist.backup" "/Volumes/YourMainVolume/System/Library/CoreServices/SystemVersion.plist"


and hit ↵.
Enter exit, hit ↵, quit Terminal and restart your Mac.

If you didn't make a backup of your SystemVersion.plist enter:

vi "/Volumes/YourMainVolume/System/Library/CoreServices/SystemVersion.plist"    


and revert the edits you made previously. (vi Quick reference)

Quit Terminal and restart your Mac.