Remove `Array#sum` method before override it

To suppress warning ('warning: method redefined; discarding old sum')
remove the method before override it.
Signed-off-by: NJeremy Daer <jeremydaer@gmail.com>

activesupport/lib/active_support/core_ext/enumerable.rb

@@ -112,6 +112,8 @@ def sum(identity = 0)
 # just calling the compat method in the first place.
 if Array.instance_methods(false).include?(:sum) && !(%w[a].sum rescue false)
   class Array
+    remove_method :sum
+
     def sum(*args) #:nodoc:
       # Use Enumerable#sum instead.
       super