Fixed reference to isURL validator function

https://github.com/chriso/validator.js/blob/master/validator.js#L164

It was hard to find the specific reason for my `type: 'url'` model attribute in sails failing since the error message was saying that the validation failed, not that the validator function was undefined or something like that. I then tried putting `type: 'caca'` in the attribute and it still threw that the value for the field was wrong instead of saying that `caca` is not a valid type hahaha.

I'm not sure if other changes are needed. Please advice.