Fixes session storage for HHVM (and possibly strict PHP environments)

The [SessionHandlerInterface](http://php.net/manual/en/class.sessionhandlerinterface.php) expects boolean return values for all methods (except read). This must've been an oversight when implementing this interface.

Unfortunately, in HHVM, this produces this (extremely unhelpful) error:

```
Warning: Failed to write session data (user). Please verify that the current setting of session.save_path is correct (unix:///var/run/redis/redis.sock) in ...
```

Quick and easy win!