use https for installing Rust

[vks]

Now that rust-lang/rust#16123 has been implemented, the website can be updated to use

$ curl https://static.rust-lang.org/rustup.sh | sudo sh

instead of

$ curl http://www.rust-lang.org/rustup.sh | sudo bash

This provides more security, although man-in-the-middle attacks are still possible as long as the website is served over http. To be able to fix this, Github needs to implement https for user pages first: isaacs/github#156

[wycats]

This is done 😄