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-908 - Support for nested field references in group operations.

We now allow referring to nested field expressions if the root segment of the nested field expression was exposed in earlier stages of the aggregation pipeline.

Original pull request: #167.