add an option to specify the ssl version used for cargo

[guanqun]

For some reason, the current cargo (1.31.0 and 1.32.0) would use TLSv1.3 here, but due to the network issue (probably behind the corporate proxy), it fails. Rust verion 1.30.0 works fine as it uses TLSv1.2.

So I'd like to add a option in "http" that specifies the ssl version to use. With this alexcrichton/curl-rust#252 fixed, I think it's quite easy to add.

If you agree this is a good option, I can file a PR, (probably need to ask for a version bump for curl-rust first).