Update library/registry image to v2.1.0

Add new tag "2.1", and update tag "2" to point at this new release.

These tags now point to distribution-library-image, which is a slimmed
down image, rather than the golang-derived image used to build registry
from source.

Signed-off-by: Aaron Lehmann <[email protected]>

library/registry

@@ -1,11 +1,11 @@
 # maintainer: Stephen Day <[email protected]> (@stevvooe)
 # maintainer: Olivier Gambier <[email protected]> (@dmp42)
 # maintainer: Richard Scothern <[email protected]> (@RichardScothern)
+# maintainer: Aaron Lehmann <[email protected]> (@aaronlehmann)
 
 latest: git://github.com/docker/[email protected]
 0.8.1: git://github.com/docker/[email protected]
 0.9.1: git://github.com/docker/[email protected]
 
-2: git://github.com/docker/distribution@1341222284b3a6b4e77fb64571ad423ed58b0d34
-2.0: git://github.com/docker/distribution@1341222284b3a6b4e77fb64571ad423ed58b0d34
-2.0.1: git://github.com/docker/distribution@1341222284b3a6b4e77fb64571ad423ed58b0d34
+2: git://github.com/docker/distribution-library-image@d5ee820f89fc8ca7b66bd88f70554e89319808e0
+2.1: git://github.com/docker/distribution-library-image@d5ee820f89fc8ca7b66bd88f70554e89319808e0