Add base64 / mutex_m dependencies for Rails 6.1

These are required for Ruby 3.4 support, which extracts `base64` /
`mutex_m` into bundled gem. The maintained versions of Rails have the
dependencies updated.

Author: voxik

gemfiles/Gemfile.rails-6.1.x

@@ -8,6 +8,8 @@ gem 'test_declarative', '0.0.6'
 gem 'rake'
 gem 'minitest', '~> 5.14'
 gem 'racc'
+gem 'base64'
+gem 'mutex_m'
 
 platforms :mri do
   gem 'oj'