Turn on hyper backend by default

[brson]

I think it works! Let's turn it on and see if anything goes bad. Should be a matter of changing the download_file_ function to prefer hyper to curl.

It would be nice if native_tls was published to crates.io though.

Once the hyper backend is live for a month or so we can disable the curl backend completely by turning off the feature of the download crate. For now I don't want to remove curl support from the download crate because I still have the notion to publish it separately as a learning tool.

[brson]

Hm, it's probably worth investigating whether there are any new revisions of native_tls, and to do a bit more due diligence by having somebody smart review the TLS integration.

[jelford]

@brson can this issue be closed in light of #993?