Error on refreshing some block types

Can't refresh BlockType table because of `protected $btIsInternal = true;`
This is the reason why upgrading from 5.5.x to 5.6.3.2 fails.
I think, we should do upgrade test more.

Ref: http://www.concrete5.org/developers/bugs/5-6-3-2/upgrade-from-5.6.2.1-fail-on-guestbook-refresh/