updated AR 4.1 reference [skip ci]

Author: mceachen

README.md

@@ -56,7 +56,7 @@ for a description of different tree storage algorithms.
 
 ## Installation
 
-Note that closure_tree only supports ActiveRecord 4.1 and later, and has test coverage for MySQL, PostgreSQL, and SQLite.
+Note that closure_tree only supports ActiveRecord 4.2 and later, and has test coverage for MySQL, PostgreSQL, and SQLite.
 
 1.  Add `gem 'closure_tree'` to your Gemfile