Only enqueue js if the widget is in use

I haven't tested it, but this should work.

Jetpack's Display Posts widget ID is `jetpack_display_posts_widget`:
https://github.com/Automattic/jetpack/blob/master/modules/widgets/wordpress-post-widget.php#L23

Read more about it here:
http://codex.wordpress.org/Function_Reference/is_active_widget