Code cleanup: Can we remove the nested functions pattern of util.assertFns?

Ref: https://github.com/jquery/globalize/pull/351/files#r21450932

---

util.assertCurrencyParameter( assert, "currency", function( invalidValue ) {
 return function() {
    Globalize.currencyFormatter( invalidValue );
  };
});

@jzaefferer

I guess we've discussed this before as well, still doesn't make much sense: Why does this test have to return two nested functions?

@rxaviers

Yeap, we have discussed about this and it was clear this was needed. Although, I also believe this could be simplified.