Bump mutex_m from 0.1.2 to 0.2.0

Bumps [mutex_m](https://github.com/ruby/mutex_m) from 0.1.2 to 0.2.0.
- [Release notes](https://github.com/ruby/mutex_m/releases)
- [Commits](ruby/mutex_m@v0.1.2...v0.2.0)

---
updated-dependencies:
- dependency-name: mutex_m
  dependency-type: indirect
  update-type: version-update:semver-minor
...

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

Gemfile.lock

@@ -170,7 +170,7 @@ GEM
     mocha (2.1.0)
       ruby2_keywords (>= 0.0.5)
     msgpack (1.7.2)
-    mutex_m (0.1.2)
+    mutex_m (0.2.0)
     net-imap (0.4.3)
       date
       net-protocol