Bump mysql2 from 0.5.5 to 0.5.6 (#777)

Bumps [mysql2](https://github.com/brianmario/mysql2) from 0.5.5 to 0.5.6.
- [Release notes](https://github.com/brianmario/mysql2/releases)
- [Commits](brianmario/mysql2@0.5.5...0.5.6)

---
updated-dependencies:
- dependency-name: mysql2
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Gemfile.lock

@@ -191,7 +191,7 @@ GEM
     msgpack (1.7.2)
     multi_xml (0.6.0)
     mutex_m (0.2.0)
-    mysql2 (0.5.5)
+    mysql2 (0.5.6)
     net-imap (0.4.9.1)
       date
       net-protocol