beta: Fix unset $lang from MWMultiVersion::setSiteInfoForWiki()

Fix "Notice: Undefined variable: lang in
/srv/mediawiki/multiversion/MWMultiVersion.php on line 187" by adding
a case for the special beta.wmflabs.org vhost and aborting if the
hostname doesn't match any known pattern.

Change-Id: Ic52df0066a2d02fb34b0a48f01658f8396ce109b