Avoid using a glob in jar_path

In attempting to include therubyrhino in a compiled .jar file (using [warbler](https://github.com/jruby/warbler)), I ran into an issue with the `glob` call in `jar/rhino/jar_path.rb`.  It seems that `glob` does not behave as expected inside a .jar file, and apparently also exhibits inconsistent behavior between JRE 6 and JRE 7.  This pull request removes the glob and constructs the name of the jar by using the gem's version.