Replacing PHPass with password_compat

Per https://github.com/owncloud/core/issues/10050#issuecomment-51333177

also https://github.com/owncloud/3rdparty/pull/110
> if the password is using the old method it will always need rehashing

That is exactly my point. You already know that it needs to be rehashed, thus calling `password_needs_rehash($storedHash, PASSWORD_BCRYPT)` is pointless (though I assumed it might also return false in some cases).