Move Elastica to vendor/

We were loading classes like Elastica/Transport/Memcache twice before this
change, but the problem didn't show up until we started using php5-memcache
which includes its own Memcache class. Moving Elastica under vendor/ avoids
Symfony1 from autoloading it, as we don't need it because it's being loaded
via the Symfony2 namespace loader (see ProjectConfiguration.class.php).