Upgrade rmp-serde to fix vulnerability

[jqnatividad]

First off, thanks for this crate @dapper91 !

I just got a GH dependabot alert regarding rmp-serde:

GHSA-255r-3prx-mf99

Bumping it to the latest release should fix it.

[dapper91]

@jqnatividad Hi. Thanks for notification.

[jqnatividad]

And thanks for the quick turnaround @dapper91 !