Switch back to JRE 6 for discovery.commons

SLING-6837 increased the parent version from 26 to 30 and with that uped the JRE version from 6 to 7. To simplify maintenance for older (eg AEM) users of discovery it would be good to stay with JRE 6 as long as feasible.

To achieve this, discovery.commons should just overwrite the sling.java.version property back to 6.