Allowing jruby-complete to use regular filesystem for fxml fetch

Currently, if jruby is running in a jar (as with jruby-complete) jrubyfx will attempt to find all fxml resources within that jar. This does not work if, for example, you install your application as an external gem and attempt to run in through jruby-complete.

This commit allows fxml fetching to fall back to the normal file system if the fxml resources are not found in the jar file.