Fix for problem with the nightlies. [1820]

This might be another one where you’ll want to run “ant clean”…sorry,
folks!

The problem appears to be caused by gluegen-2.2.1.jar looking for
native gluegen-rt libraries within gluegen-2.2.1-natives-*-*.jar
(though it seems like it should be looking within
gluegen-rt-2.2.1-natives-*-*.jar instead).