build: Require Vala >= 0.28.0.16
build: Require Vala >= 0.28.0.16

GTK+'s VAPI has changed and build is broken against latest Vala until we
adapt to latest Gtk+ VAPI and we can't adapt to that change while
maintaining compatibility with older Vala. Hence this change.

https://bugzilla.gnome.org/show_bug.cgi?id=749914

vbenes/gnome-boxes,vbenes/gnome-boxes