Bump minitest from 5.20.0 to 5.21.1

Bumps [minitest](https://github.com/minitest/minitest) from 5.20.0 to 5.21.1.
- [Changelog](https://github.com/minitest/minitest/blob/master/History.rdoc)
- [Commits](minitest/minitest@v5.20.0...v5.21.1)

---
updated-dependencies:
- dependency-name: minitest
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -262,7 +262,7 @@ GEM
     mini_magick (4.11.0)
     mini_mime (1.1.5)
     mini_portile2 (2.8.5)
-    minitest (5.20.0)
+    minitest (5.21.1)
     money (6.16.0)
       i18n (>= 0.6.4, <= 2)
     msgpack (1.7.2)