Turn on rustls when compiling using Cross in Travis

[r-darwish]

Cross dropped OpenSSL from their images, which causes Travis to break. I think that compiling with rustls will fix that.

[jaemk]

That's... interesting. I'll merge your recent #46 in the meantime since it looks benign. I guess we should switch to defaulting to rustls and add an openssl feature instead

[pojntfx]

For those which struggle to get cross-compilation working, I've found the following to work:

self_update = { version = "0.27.0", features = ["rustls"], default-features = false }