Make the nightly toolchain version available?

[alecmocatta]

A couple of times I've been bitten by crates that compile on the nightly I have locally but not on the older/younger nightly used by docs.rs.

It would be cool if the docs.rs nightly version, i.e. currently nightly-2019-07-21, was available somewhere so I could catch these sorts of failures in my CI tests rather than get broken docs.rs builds.

This is how I envisage using it in my CI test prior to publishing:

TOOLCHAIN=$(curl -s -f https://docs.rs/toolchain)
rustup toolchain install $TOOLCHAIN
cargo +$TOOLCHAIN doc

[alecmocatta]

Fixed by #407