We don't know for sure the next version will be 6

While I believe 6 is the next planned release, plans can change.

Gemfile

@@ -17,7 +17,7 @@ platforms :jruby do
   if ENV['RAILS_VERSION'] == 'edge'
     gem 'railties', :github => 'rails/rails'
   else
-    gem 'railties', (ENV['RAILS_VERSION'] || ['>= 3.0', '< 6.0'])
+    gem 'railties', (ENV['RAILS_VERSION'] || ['>= 3.0', '< 5.3'])
   end
 end
 
@@ -30,8 +30,8 @@ group :test do
     gem 'actionmailer', :github => 'rails/rails'
     gem 'activerecord', :github => 'rails/rails'
   else
-    gem 'actionmailer', (ENV['RAILS_VERSION'] || ['>= 3.0', '< 6.0'])
-    gem 'activerecord', (ENV['RAILS_VERSION'] || ['>= 3.0', '< 6.0'])
+    gem 'actionmailer', (ENV['RAILS_VERSION'] || ['>= 3.0', '< 5.3'])
+    gem 'activerecord', (ENV['RAILS_VERSION'] || ['>= 3.0', '< 5.3'])
   end
 
   gem 'coveralls', :require => false

delayed_job.gemspec

@@ -1,7 +1,7 @@
 # -*- encoding: utf-8 -*-
 
 Gem::Specification.new do |spec|
-  spec.add_dependency 'activesupport', ['>= 3.0', '< 6.0']
+  spec.add_dependency 'activesupport', ['>= 3.0', '< 5.3']
   spec.authors        = ['Brandon Keepers', 'Brian Ryckbost', 'Chris Gaffney', 'David Genord II', 'Erik Michaels-Ober', 'Matt Griffin', 'Steve Richert', 'Tobias Lütke']
   spec.description    = 'Delayed_job (or DJ) encapsulates the common pattern of asynchronously executing longer tasks in the background. It is a direct extraction from Shopify where the job table is responsible for a multitude of core tasks.'
   spec.email          = ['brian@collectiveidea.com']