build: Require Vala >= 0.18.0
build: Require Vala >= 0.18.0

Vala 0.16 has a bug in the g_dbus_message_get_body binding.

hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker