Fix fetch and install tasks

* I changed the way that deletion of the installation tarball works; it wasn't checking if the file existed before it tried to delete it.
* `jettywrapper` uses the ZIP_URL variable, and we're overriding it to download a specific version of Solr and Jetty for Blacklight
* the `to_path` method only works when the path is a `Pathname` instance. `Pathname.to_s` and `Pathname.to_path` are equivalent.