Questionable licence on BurntSushi/toml

The github.com/BurntSushi/toml which we use for config parsing uses a non-standard licence.

https://gitlab.com/gitlab-org/gitaly/blob/master/vendor/github.com/BurntSushi/toml/COPYING

@ernstvn can you confirm that this is an acceptable licence (bearing in mind that this is a very popular library and one that would be tricky to remove)