bugfix: memcache extension wasn't loaded
bashizm: http://mywiki.wooledge.org/Bashism
Silence memcache install also for php 5.5 builds
https://github.com/yiisoft/yii/pull/927
https://github.com/yiisoft/yii/pull/931
Merge pull request #2124 from arvenil/fix-memcache-install-for-php-5.5

Fix memcache install for php 5.5 builds in travis