Fix error in class-mailchimp.php

git-svn-id: https://plugins.svn.wordpress.org/mailchimp-for-wp/trunk@1006844 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update readme.txt, update to version 2.1.5 is not thát severe.

git-svn-id: https://plugins.svn.wordpress.org/mailchimp-for-wp/trunk@1006846 b8457f37-d9ea-0310-8a92-e5e31aec5664