Minimum Rust badge seems incorrect

[Voultapher]

Just tried building the cranelift tests with cd cranelift && cargo test --all using Rust 1.44 and it failed building one of the dependencies because it wanted to use this dtolnay/proc-macro2#228. After upgrading to 1.47 everything works as expected. The top level README has a Rust 1.37+ badge. Either that is wrong or misleading.

Edit: Actually I need nightly to build and run the tests. I suggest we put that in the relevant README.

[alexcrichton]

Ah yes I think that's a mistake, sorry about that! A PR would be welcome to clarify and update this!

[Voultapher]

This issue hasn't seen activity in long time so I'm closing it.