GNOME Builder is under heavy development. This usually implies that such an application might require very new versions of its dependencies.
Upstream recently bumped their dependencies, and now require things that are only in Rawhide.
I have no intention to provide development builds of Gtk3 (among other things) in a Fedora 21 copr, as that might imply either breaking half of the distro, or having to rebuild it.
As a result, the GNOME Builder copr will from now on be Rawhide-only.
I have dropped the Fedora 21 repos, they won't be updated any more.
If you were using it on Fedora 21, please delete it:
# rm -f /etc/yum.repos.d/_copr_bochecha-gnome-builder.repo
If you still want to try GNOME Builder on Fedora 21, you'll now have to go the jhbuild route.