Keep consistency on allow_blank and allow_nil qualifiers

After some discussion[1] was decided that these qualifiers do not accept
any parameter.

thoughtbot#722

lib/shoulda/matchers/active_model/validate_inclusion_of_matcher.rb

@@ -302,12 +302,12 @@ def in_range(range)
           self
         end
 
-        def allow_blank(allow_blank = true)
+        def allow_blank
           @options[:allow_blank] = allow_blank
           self
         end
 
-        def allow_nil(allow_nil = true)
+        def allow_nil
           @options[:allow_nil] = allow_nil
           self
         end