osgi: rely on naming convention to find the JRuby bundle

The JRuby bundle is a simple OSGI bundle, it may make sense to find it
among other platform bundles (under {killbill.osgi.bundle.install.dir}/platform/).

To avoid yet another configuration parameter, rely on a naming
convention to find it (jruby.jar).

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>