Changed URL to WP core arrow images for widget admin side due to 3.8 no longer having the arrows-vs.png file. Changed to arrows.png file that exists with both admin themes pre 3.8 and post 3.8. Trac fixes #2083

git-svn-id: https://plugins.svn.wordpress.org/benchmark-email-lite/trunk@821672 b8457f37-d9ea-0310-8a92-e5e31aec5664