move Hash monkeypatch to separate file

lib/cask/extend.rb

@@ -1,3 +1,4 @@
 # monkeypatching
+require 'cask/extend/hash'
 require 'cask/extend/string'
 require 'cask/extend/pathname'

lib/cask/extend/hash.rb

@@ -0,0 +1,8 @@
+class Hash
+  def assert_valid_keys(*valid_keys)
+    unknown_keys = self.keys - valid_keys
+    unless unknown_keys.empty?
+      raise CaskError.new %Q{Unknown keys: #{unknown_keys.inspect}. Running "#{UPDATE_CMD}" will likely fix it.}
+    end
+  end
+end

lib/cask/utils.rb

@@ -23,16 +23,6 @@ def utf8_inspect
   end
 end
 
-# monkeypatch Hash
-class Hash
-  def assert_valid_keys(*valid_keys)
-    unknown_keys = self.keys - valid_keys
-    unless unknown_keys.empty?
-      raise CaskError.new %Q{Unknown keys: #{unknown_keys.inspect}. Running "brew update && brew upgrade brew-cask && brew cleanup && brew cask cleanup" will likely fix it.}
-    end
-  end
-end
-
 class Buffer < StringIO
   def initialize(tty = false)
     super()