Switching from Java-1.6 to Java-1.7 for WatchDog

Support for Java 1.6 has been cancled in February 2013 (see
http://itssb.web.cern.ch/service-change/linux-oracle-sun-java-6-16-end-life-notice-february-2013/15-01-2013).
We therefore switch to Java 1.7 as the default JDK.

Refs #6