Fixed a bug where menu items with same position as GradeBook would be overwritten

git-svn-id: https://plugins.svn.wordpress.org/an-gradebook/trunk@921408 b8457f37-d9ea-0310-8a92-e5e31aec5664
tagging v2.4.2

git-svn-id: https://plugins.svn.wordpress.org/an-gradebook/trunk@921409 b8457f37-d9ea-0310-8a92-e5e31aec5664