Bump msgpack from 1.7.3 to 1.7.4

Bumps [msgpack](https://github.com/msgpack/msgpack-ruby) from 1.7.3 to 1.7.4.
- [Changelog](https://github.com/msgpack/msgpack-ruby/blob/master/ChangeLog)
- [Commits](msgpack/msgpack-ruby@v1.7.3...v1.7.4)

---
updated-dependencies:
- dependency-name: msgpack
  dependency-type: indirect
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -151,7 +151,7 @@ GEM
     minitest (5.25.1)
     mocha (2.5.0)
       ruby2_keywords (>= 0.0.5)
-    msgpack (1.7.3)
+    msgpack (1.7.4)
     net-http (0.5.0)
       uri
     net-imap (0.5.0)