Fallback to old Mongo() class if MongoClient() is not found

This is related to https://github.com/colinmollenhour/mongodb-php-odm/pull/44.
On dev machines it's easy to get the last MongoDB PHP driver but on production servers, this PHP driver is not necessary at the last version.
This small fix allows to use either MongoClient() or Mongo() as a fallback.
Merge pull request #49 from semalead/fallback-to-mongo-class

Fallback to old Mongo() class if MongoClient() is not found