Fix activejob integration for Sidekiq 6

Sidekiq::MAJOR was added in sidekiq/sidekiq@b4092e3, which is only
included in 7.0.0+ so we can't use it to check the version for
Sidekiq 6.

Since the test code is written to support both, the condition should
also support both.

activejob/test/support/integration/adapters/sidekiq.rb

@@ -54,7 +54,7 @@ def start_workers
 
       require "sidekiq/cli"
       require "sidekiq/launcher"
-      if Sidekiq::MAJOR >= 7
+      if Gem::Version.new(Sidekiq::VERSION) >= Gem::Version.new("7")
         config = Sidekiq.default_configuration
         config.queues = ["integration_tests"]
         config.concurrency = 1