Remove support for lower than AppStream 0.8
Remove support for lower than AppStream 0.8

Even Fedora 21 is using v0.8 metadata, and this lets us clean up some code.

hughsie/appstream-glib,ikeydoherty/appstream-glib,mitya57/appstream-glib,ximion/appstream-glib,hughsie/appstream-glib,superm1/appstream-glib,superm1/appstream-glib,mitya57/appstream-glib,hughsie/appstream-glib,ikeydoherty/appstream-glib,ximion/appstream-glib,ximion/appstream-glib