Fix for https://issues.apache.org/jira/browse/OPTIQ-269

Detects the version of mongod to determine which kind of intermediate
result to utilize.

For versions 2.6+, a Cursor result will be returned form the
DBCollection otherwise a AggregationOutput result is returned.

An Iterator<DBObject> object is retrieved from the intermediate result 
and returned by MongoTable.aggregate.

Updated dependency in for mongo-java-driver in master/mongodb/pom.xml:

    <dependency>
      <groupId>org.mongodb</groupId>
      <artifactId>mongo-java-driver</artifactId>
      <version>2.12.3</version>
    </dependency>