Simplify .travis.yml

* APC, Mongo and Memcache are [preinstalled by Travis](http://about.travis-ci.org/docs/user/languages/php/#Preinstalled-PHP-extensions);
* `phpenv config-add` is the [preferred way](http://about.travis-ci.org/docs/user/languages/php/#Custom-PHP-configuration) to modify the configuration
* install APCu from PECL (PHP 5.5), do not download it directly