memcache: Check for existence of memcache extension.

The old test was faulty, since PHP would fail before reaching it. This
patch adds a specific test for the Memcache class before continuing.

Thanks to Thijs Kinkhorst for reporting this bug!

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3230 44740490-163a-0410-bde0-09ae8108e29a