check redis for :use_rack_cache

Gemfile.lock

@@ -42,7 +42,7 @@ GIT
 
 GIT
   remote: git://github.com/travis-ci/travis-core.git
-  revision: 3d9b40dd4bdf3a4f7c825faddd2ca88269092a03
+  revision: 2e3072e862709c5a1a3daf51abb13adc518e3f14
   branch: sf-rich-states
   specs:
     travis-core (0.0.1)

lib/travis/api/app.rb

@@ -62,7 +62,8 @@ module Travis::Api
         use ActiveRecord::ConnectionAdapters::ConnectionManagement
         use ActiveRecord::QueryCache
 
-        if memcache_servers = ENV['MEMCACHE_SERVERS']
+        memcache_servers = ENV['MEMCACHE_SERVERS']
+        if Travis::Features.feature_active?(:use_rack_cache) && memcache_server
           namespace = File.read('.deploy-sha')[0..7]
           use Rack::Cache,
             verbose: true,