Add mutex_m gem as dependency in rails 6.1 tests

This patch will fix https://github.com/ohler55/oj/actions/runs/7908156404/job/21586745798

The mutex_m gem will be changed bundled gem since Ruby 3.4.0.
To use bundled gem, it requires to specify the dependency in Gemfile.

Ref. #901

gemfiles/rails_6.1.gemfile

@@ -4,5 +4,6 @@ source 'https://rubygems.org'
 
 gem 'rails', '6.1.3.1'
 gem 'sqlite3'
+gem 'mutex_m'
 
 gemspec :path => '../'