[tests] Fix gotcha with new activesupport redis versions

This fixes the error:

    uninitialized constant ActiveSupport::VERSION

when loading active_support/cache/redis_store

Author: ktheory

spec/integration/offline_spec.rb

@@ -1,5 +1,5 @@
 require 'active_support/cache'
-require 'active_support/cache/redis_store'
+require 'redis-activesupport'
 require 'dalli'
 require_relative '../spec_helper'