Upgrade smallvec to fix insert_many vulnerability.

[jsirois]

See: https://rustsec.org/advisories/RUSTSEC-2021-0003

[ci skip-build-wheels]

[jsirois]

The issue was surfaced in CI CRON: https://travis-ci.com/github/pantsbuild/pants/builds/212643465?utm_medium=notification&utm_source=email and I was the lucky winner of the CI failure email.

The change was mechanical:

$ ./cargo update --package smallvec
error: There are multiple `smallvec` packages in your project, and the specification `smallvec` is ambiguous.
Please re-run this command with `-p <spec>` where `<spec>` is one of the following:
  smallvec:0.6.13
  smallvec:1.5.1
$ ./cargo update -p smallvec:0.6.13
    Updating crates.io index
    Updating smallvec v0.6.13 -> v0.6.14
$ ./cargo update -p smallvec:1.5.1
    Updating crates.io index
    Updating smallvec v1.5.1 -> v1.6.1

[Eric-Arellano]

Thanks!