Delete vfb-settings when uninstalling

git-svn-id: https://plugins.svn.wordpress.org/visual-form-builder/trunk@755648 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update stable tag to 2.7.8

git-svn-id: https://plugins.svn.wordpress.org/visual-form-builder/trunk@755650 b8457f37-d9ea-0310-8a92-e5e31aec5664