Change contributing documentation to reflect that Volta uses the late…

[charlespierce]

Info

• In https://github.com/volta-cli/volta/pull/868, we are using the matches! macro, which was introduced in Rust 1.42
• The docs currently indicate that Volta is intended to compile on Rust 1.41 or newer, so we would need to bump that number.
• Since Volta is a binary project and not a library, there isn't any external reason why would need to support building against an older Rust version.
• In our CI, we are currently using the latest stable for each build.

Changes

• Updated the documentation to reflect that Volta is intended to be compiled with the latest stable Rust release.

[chriskrycho]

Seems good to me!