Revert: glib >= 2.41.2 and valac < 0.25.2 not compatible

Since glib-2.0.vapi from Vala 0.26 has been imported the glib >= 2.41.2
and valac < 0.25.2 incompatibility is fixed.

http://valajournal.blogspot.cz/2014/12/vala-deadlocks-with-glib-242.html

This reverts commit 9ce32b46ca7272a5c4184fdfd3cdd72d47c5a0dd.