Update build.yaml to build 3.X/java6 on commit
Update build.yaml to build 3.X/java6 on commit

Motiviation:

To get more immediate feedback about possible JDK incompatibilities,
include openjdk6 in build matrix for commit builds.

Modifications:

Update build.yaml to build 3.X java6 on commit

Result:

Cassandra 3.X w/ openjdk6 included in on commit build matrix.

tolbertam/java-driver,tolbertam/java-driver,tolbertam/java-driver