Aliasing "must_not" just like we alias "must"

Signed-off-by: Luke Kanies <luke@madstop.com>

spec/monkey_patches/alias_should_to_must.rb

@@ -2,4 +2,5 @@ class Object
     # This is necessary because the RAL has a 'should'
     # method.
     alias :must :should
+    alias :must_not :should_not
 end