Skip loading skins if no SkinFactory

Versions of MediaWiki without SkinFactory just blow up with mediawiki-vagrant
now because it attempts to load vector (and any other skins) using
SkinFactory.  Rather than blow up on versions of MediaWiki with SkinFactory
we should just not load new skins with them.

Change-Id: Ic704e2e71023b00e8f480d197f3a15909bd7cdf2