replace deprecated 'Mongo' class usage

[rocksfrow]

I figured I would open an issue for this as it's definitely something that should at least be on the radar.

The Mongo class is currently used for the connection to Mongo. This class extends MongoClient, and has been deprecated (http://php.net/manual/en/class.mongo.php).

"This class has been DEPRECATED as of version 1.3.0. Relying on this feature is highly discouraged. Please use MongoClient instead. "

The replacement of Mongo with MongoClient should be simple, the bigger part would be verifying/replacing any other functions being used from the defunct Mongo class (if any).

NOTE: I wouldn't mind tackling this if my work will get merged.

[rocksfrow]

OK, I verified none of the deprecated methods are being used, so we just need to replace Mongo with MongoClient usage.

/* Methods */
protected bool connectUtil ( void )
public static int getPoolSize ( void )
public string getSlave ( void )
public bool getSlaveOkay ( void )
public array poolDebug ( void )
public static bool setPoolSize ( int $size )
public bool setSlaveOkay ([ bool $ok = true ] )
public string switchSlave ( void )

[rocksfrow]

@nicktacular I'm going to close this issue because it's a super small change, and I've made it in my fork. I am making other more significant changes for replica set support which I've created a separate issue for.

[rocksfrow]

Here is the change if interested: https://github.com/rocksfrow/php-mongo-session/commit/ba273cfe261d0167dd52fecfa4020c923d7151fd

[nicktacular]

Please issue a pull request and I'd be glad to pull in some of your changes.