DATAMONGO-895, DATAMONGO-896 - Assert compatibility with latest MongoDB Java driver.

Upgrade next MongoDB driver version to 2.12.0. Strong upgrade coming in a subsequent commit to make sure we can backport the compatibility checks to the bugfix branch without forcing users into a driver upgrade.

Relaxing error message comparison in assertion so that it still matches against the message returned by MongoDB 2.6. When comparing the value of the version field, compare against a Long rather than an Integer, since the version field generated is a Long. This allows the test to pass against the upcoming 2.12.0 release of the Java driver, which has a stricter implementation of BasisDBObject.equals(…).

Original pull requests: #159, #160.
DATAMONGO-910 - Upgraded to latest MongoDB Java driver 2.12.0.

Removed special mongo-osgi property as the driver version is now a valid OSGi version number.