Forcing Java 6 source and target compatibility

This change ought to force Java 6 compatible bytecode to come out of the build.  However, keep in mind that building with a non-JDK 6 compiler always runs the risk of including JDK 7/8/etc. specific class references if they're in your source that don't exist in JDK 6, so its not a failsafe stand-in for building on a true JDK target.