Fix CI against bundler's master

Current run seems to be failing. I'm not sure exactly why, but using a
"more standard" trick for redefining the method seems to fix it.

lib/rubygems/core_ext/kernel_warn.rb

@@ -6,12 +6,10 @@
   module Kernel
     path = "#{__dir__}/" # Frames to be skipped start with this path.
 
-    # Suppress "method redefined" warning
-    original_warn = instance_method(:warn)
-    Module.new {define_method(:warn, original_warn)}
-
     original_warn = method(:warn)
 
+    remove_method :warn
+
     class << self
 
       remove_method :warn