[Fix] URL generator using absolute path. Fixes #761

This is a fix for #761. Currently if you have L4 installed into a sub-directory, e.g., `localhost/laravel` then all generated URLs that use `URL::route()` (this also applies to `URL::action()`) will have `laravel/laravel` prefixed instead of just a single `laravel`.

It seems the Symfony `UrlGenerator` just needed to generate as a relative path.