README: rm references to old Ubuntu distros

Support for Ubuntu 11.04 officially ended on 28 October 2012.

Support for Ubuntu Oneiric Ocelot was officially ended on 9 May 2013.

Remove the references to these EOL distro versions.

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>

README.md

@@ -34,8 +34,7 @@ The list of debian packages dependencies can be installed with:
 	./install-deps.sh
 
 Note: libsnappy-dev and libleveldb-dev are not available upstream for
-natty, oneiric, and squeeze.  Backports for Ceph can be found at
-ceph.com/debian-leveldb.
+Debian Squeeze.  Backports for Ceph can be found at ceph.com/debian-leveldb.
 
 rpm-based
 ---------