Bump mysql2 from 0.5.5 to 0.5.6

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:production
  update-type: version-update:semver-patch
...

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

Gemfile.lock

@@ -268,7 +268,7 @@ GEM
       ruby2_keywords (>= 0.0.5)
     msgpack (1.7.2)
     multi_json (1.15.0)
-    mysql2 (0.5.5)
+    mysql2 (0.5.6)
     net-imap (0.4.10)
       date
       net-protocol