[dm-validations] :format validator now works with fixnums

* Always calling #to_s on the value to validate leads to
  4 spec errors, therefore why I added the check if value
  is_a?(Fixnum)

[#859 state:resolved]

dm-validations/lib/dm-validations/validators/format_validator.rb

@@ -34,7 +34,7 @@ def call(target)
 
         valid = case validator
           when Proc   then validator.call(value)
-          when Regexp then value =~ validator
+          when Regexp then (value.is_a?(Fixnum) ? value.to_s : value) =~ validator
           else
             raise UnknownValidationFormat, "Can't determine how to validate #{target.class}##{field_name} with #{validator.inspect}"
         end