Fix redundant Util:: namespacing

On line 5 the reference to `Util` resolves to the class that was just opened on line 4, so `Util::` is redundant.

lib/webmock/util/hash_counter.rb

@@ -2,7 +2,7 @@
 
 module WebMock
   module Util
-    class Util::HashCounter
+    class HashCounter
       attr_accessor :hash
       def initialize
         self.hash = {}