1.30.1 missing in master RELEASES.md

[tspiteri]

https://github.com/rust-lang/rust/blob/master/RELEASES.md contains a section for 1.31.0, but no section for 1.30.1.

[pietroalbini]

Woops, thanks for pointing this out @tspiteri! Sent a PR to backport this to master (which will also be included into beta).