Release version 2.19.1: "Eight Ball Deluxe"

[rcurtin]

This automatically-generated pull request adds the commits necessary to make the 2.19.1 release.

Once the PR is merged, mlpack-bot will tag the release as HEAD~1 (so that it doesn't include the new HISTORY block) and publish it.

Or, well, hopefully that will happen someday.

When you merge this PR, be sure to merge it using a rebase.

Changelog

• Avoid deprecation warnings in Armadillo 11.2+ (#347).

[rcurtin]

It just seemed like it had been a bit too long. So, why not a patch release? :)