update java source version to 1.6

in 1.5 methods overriding an interface definitions aren't allowed to be
annotated with @override, this is a 1.6+ Java feature.

Otherwise it will produce 35 errors.

example: 
https://github.com/jidesoft/jide-oss/blob/master/src/com/jidesoft/swing/JideSwingUtilities.java#L991#

Another fix would be to delete all this 35 appearances, but I think Java 1.6 as a compile is common these days, so it will be no probelm (and other devs here seems to use 1.6 as well)