[travis ci] Improvements to the memcache setup

Per [this](http://about.travis-ci.org/blog/2013-03-08-preinstalled-php-extensions/) blog entry, some PHP extensions are installed by default and must no longer be installed via PECL. The pear/pecl cli doesn't seem to be ready for PHP v5.5 (cf [bug #63073](https://bugs.php.net/bug.php?id=63073)), so memcache.so will still be compiled for some time.

Also: `memcached` has been removed from the services section. Turned out it's been started already in the VMs for some time now.