Updates for WordPress 4.0

git-svn-id: https://plugins.svn.wordpress.org/thumbnail-field/trunk@992671 b8457f37-d9ea-0310-8a92-e5e31aec5664
Typo in readme

git-svn-id: https://plugins.svn.wordpress.org/thumbnail-field/trunk@992672 b8457f37-d9ea-0310-8a92-e5e31aec5664