configure.ac: post release version bump to 3.15.1
configure.ac: post release version bump to 3.15.1

sfeltman/pygobject,GNOME/pygobject,pexip/pygobject,pexip/pygobject,pexip/pygobject,GNOME/pygobject,sfeltman/pygobject,GNOME/pygobject,sfeltman/pygobject