use nickel's rust-url version

This fixes the errors on nickel's build:
https://travis-ci.org/nickel-org/nickel.rs/builds/40414397
```bash
src/query_string.rs:77:19: 77:43 error: mismatched types: expected `url::Url`, found `url::Url` (expected struct url::Url, found a different struct url::Url)
```