fix breaking change in cargo-audit

[bmc-msft]

Address a breaking change introduced in cargo-audit 0.13.1.

For more info, see rustsec/rustsec#280

[bmc-msft]

Note, this also includes an update of rust prereqs.

Crate:         futures-util
Version:       0.3.6
Title:         MutexGuard::map can cause a data race in safe code
Date:          2020-10-22
ID:            RUSTSEC-2020-0059
URL:           https://rustsec.org/advisories/RUSTSEC-2020-0059
Solution:      Upgrade to >=0.3.7