gtk2: follow prepend DLL path method name change
gtk2: follow prepend DLL path method name change

[ruby-gnome2-devel-en] BUG: undefined method
`prepend_environment_path' for GLib:Module

Reported by Detlef Reichl. Thanks!!!

kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2