Fix Ruby 2.5/2.6 CI by limiting concurrent Ruby version

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.

This borrows the same fix used in Sequel's CI.

Author: jeremyevans

.ci.gemfile

@@ -14,6 +14,9 @@ if RUBY_VERSION < '2.2'
   gem 'activerecord', '< 5'
 elsif RUBY_VERSION < '2.3'
   gem 'activerecord', '< 5.2'
+elsif RUBY_VERSION < '2.7'
+  gem 'activemodel', '< 7'
+  gem 'concurrent-ruby', '1.3.4'
 else
   gem 'activerecord'
 end