RUSTSEC-2020-0077: memmap is unmaintained

[SamuelQZQ]

The memmap is unmaintained. Maybe we should replace it with memmap2?

[SamuelQZQ]

@wesleywiser Maybe a stupid question, how can I know when will the new release including this change available?

[wesleywiser]

@SamuelQZQ not a stupid question at all! Thanks for the ping, I've just released 9.1.1 which has this change included.