Bump plug from 1.15.3 to 1.16.1 (#429)

Bumps [plug](https://github.com/elixir-plug/plug) from 1.15.3 to 1.16.1.
- [Changelog](https://github.com/elixir-plug/plug/blob/main/CHANGELOG.md)
- [Commits](elixir-plug/plug@v1.15.3...v1.16.1)

---
updated-dependencies:
- dependency-name: plug
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Ross Garfield <[email protected]>

mix.exs

@@ -39,7 +39,7 @@ defmodule Slax.Mixfile do
       {:ecto_sql, "~> 3.12.1"},
       {:postgrex, "~> 0.16"},
       {:plug_cowboy, "~> 2.5"},
-      {:plug, "~> 1.15.3"},
+      {:plug, "~> 1.16.1"},
       {:httpoison, "~> 1.5"},
       {:yaml_front_matter, "~> 1.0"},
       {:ex_doc, "~> 0.25", only: :dev, runtime: false},