Fixed pre-5.3.0 PHP undefined constant notice

With PHP < 5.3.0, the use of [E_DEPRECATED](http://php.net/manual/errorfunc.constants.php#errorfunc.constants.errorlevels.e-deprecated-error) and [E_USER_DEPRECATED](http://php.net/manual/errorfunc.constants.php#errorfunc.constants.errorlevels.e-user-deprecated) in `lib/Raven/Client.php` [[1]](https://github.com/getsentry/raven-php/blob/6a4031ef13f93c456b1561fa0ca30d246e4b7ef7/lib/Raven/Client.php#L541) [[2]](https://github.com/getsentry/raven-php/blob/6a4031ef13f93c456b1561fa0ca30d246e4b7ef7/lib/Raven/Client.php#L542) could throw a PHP notice under certain circumstances.