Bump to v4.2.0.

mergeWith.js

@@ -8,6 +8,8 @@ import createAssigner from './_createAssigner';
  * method instead. The `customizer` is invoked with seven arguments:
  * (objValue, srcValue, key, object, source, stack).
  *
+ * **Note:** This method mutates `object`.
+ *
  * @static
  * @memberOf _
  * @category Object