Fix upgrade fatal error due to PermissionKey defined as abstract class.
Fix upgrade fatal error due to PermissionKey defined as abstract class.

This fix eliminates a fatal error on line 77 of "concrete/core/models/permission/key.php" when upgrading from 5.2.x to 5.6.x due to attempt to instantiate an abstract class.


Former-commit-id: bbe7d48c8e42c3c38bfc378bb8688fb4b5bc08fa
MrKarlDilkington/concrete5,deek87/concrete5,acliss19xx/concrete5,biplobice/concrete5,KorvinSzanto/concrete5,concrete5/concrete5,acliss19xx/concrete5,mainio/concrete5,mainio/concrete5,olsgreen/concrete5,jaromirdalecky/concrete5,rikzuiderlicht/concrete5,deek87/concrete5,biplobice/concrete5,triplei/concrete5-8,rikzuiderlicht/concrete5,acliss19xx/concrete5,KorvinSzanto/concrete5,mlocati/concrete5,deek87/concrete5,concrete5/concrete5,triplei/concrete5-8,jaromirdalecky/concrete5,jaromirdalecky/concrete5,hissy/concrete5,haeflimi/concrete5,concrete5/concrete5,triplei/concrete5-8,haeflimi/concrete5,jaromirdalecky/concrete5,haeflimi/concrete5,mlocati/concrete5,concrete5/concrete5,a3020/concrete5,deek87/concrete5,olsgreen/concrete5,biplobice/concrete5,mlocati/concrete5,MrKarlDilkington/concrete5,haeflimi/concrete5,a3020/concrete5,olsgreen/concrete5,KorvinSzanto/concrete5,hissy/concrete5,mainio/concrete5,MrKarlDilkington/concrete5,biplobice/concrete5,mlocati/concrete5,hissy/concrete5,rikzuiderlicht/concrete5,hissy/concrete5