Fix concurrent-ruby version in Ruby 2.5/2.6 CI

active_support/logger_thread_safe_level.rb in Rails 6.1
references Logger without requiring it, which only worked
because concurrent-ruby was requiring it.  concurrent-ruby
1.3.5 dropped the logger require, which broke active_support.

.ci.gemfile

@@ -37,6 +37,7 @@ elsif RUBY_VERSION < '2.4.0'
   gem 'activemodel', '<6.0.0'
 elsif RUBY_VERSION < '2.7.0'
   gem 'activemodel', '<7.0.0'
+  gem 'concurrent-ruby', '1.3.4'
 else
   gem 'activemodel'
 end