Add development dependency on mutex_m

Minitest depends on mutex_m but doesn't declare the dependency. It
removes the dependency in newer (yet to be released) code, so lets
declare this dev dep until minitest is upgraded

Author: tenderlove

Gemfile

@@ -15,4 +15,7 @@ group :development do
   gem "rubocop", require: false
   gem "standardrb", require: false
   gem "rubocop-minitest", require: false
+
+  # FIXME: Remove after minitest removes dependency
+  gem "mutex_m"
 end