Preserve Travis memcached install

Looking at recent Travis logs, during the installation of our apt
packages we were upgrading the built-in install of memcached, and
the upgraded version was not started properly (which prevented the
test suite from running).  This change filters memcached out of the
packages to be installed by apt-get, so that we don't touch the
existing install on the Travis VM.

Also removes postgresql from "services:" block, since according to
Travis documentation, postgresql is started at boot.
Amend previous commit to allow libmemcached-dev

The caret makes sure grep matches only the beginning of a line, e.g.
packages starting with "memcached" and not all packages containing
"memcached."  This is important because pylibmc is built on
libmemcached.
Merge pull request #960 from learning-unlimited/travis-fix-memcached

Travis fix memcached