jre/lib/rt.jar doesn't exist on OSX, rather bundle/Classes/classes.jar

When running mvn eclipse:eclipse we get annoying messages that the JDK doesn't have a valid jre/lib/rt.jar. This file actually doesn't exist on OS X installations, and can be found under bundle/Classes/classes.jar. The attached patch fixes this issue, and is backwards compatible with other OS (just tries another additional location).