Made Stash FileSystem driver the default one

Since https://github.com/ezsystems/ezpublish-community/commit/74cbd1f9ce5adc07ac1604c151605e08d9c3a430, we're pointing to StashBundle and Stash lib forks, containing the right fixes for FileSystem (see https://github.com/tedivm/Stash/pull/66).

So we don't have any reasons now not to use FileSystem driver by default.