Update upgrade guide for Redis >= 2.4.0 requirement

[ci skip]

Closes #2837

README.md

@@ -71,7 +71,7 @@ GitLab is a Ruby on Rails application that runs on the following software:
 - Ubuntu/Debian/CentOS/RHEL
 - Ruby (MRI) 2.1
 - Git 1.7.10+
-- Redis 2.0+
+- Redis 2.4+
 - MySQL or PostgreSQL
 
 For more information please see the [architecture documentation](http://doc.gitlab.com/ce/development/architecture.html).

doc/update/7.14-to-8.0.md

@@ -175,12 +175,21 @@ Also note that because Apache does not support upstreams behind Unix sockets you
 Now, GitLab CE and EE has CI integrated. However, migrations don't happen automatically and you need to do it manually.
 Please follow the following guide [to migrate](../migrate_ci_to_ce/README.md) your GitLab CI instance to GitLab CE/EE.
 
-### 10. Start application
+### 10. Use Redis v2.4.0+
+
+Previous versions of GitLab allowed Redis versions >= 2.0 to be used, but
+Sidekiq jobs could fail due to lack of support for the SREM command. GitLab
+8.0 now checks that Redis >= 2.4.0 is used. You can check your Redis version
+with the following command:
+
+    redis-cli info | grep redis_version
+
+### 11. Start application
 
     sudo service gitlab start
     sudo service nginx restart
 
-### 11. Check application status
+### 12. Check application status
 
 Check if GitLab and its environment are configured correctly: