Detect OS X JDK using /usr/libexec/java_home in startGrails

On my machine, CurrentJDK is not updated to point to Java 7
when I change the Java Preferences list order.

Therefore, the current symbolic link checking method does
not work, and returns Java 6.  Using /usr/libexec/java_home
seems to work (at least for me)

See "Java Home" here:
http://developer.apple.com/library/mac/#qa/qa1170/_index.html