Force the language level of Drools to 1.6

When running with Java 7 or higher Drools will try to use that java language level, but then fail as
it is not supported until Drools 5.2.1.FINAL (see JBRULES-3163).
Updating to 5.2.1.FINAL might break other rules, so this is the "safer" approach.
Merge pull request #17 from ankon/rbcc-java7

Force the language level of Drools to 1.6