Redis 3.2.0 is minimum required version

As per commit e5aa157f, Redis 3.2.0 or
higher is required for Merge Trains to work.
Signed-off-by: Balasankar "Balu" C <balasankarc@autistici.org>

lib/system_check/app/redis_version_check.rb

@@ -5,7 +5,7 @@ require 'redis'
 module SystemCheck
   module App
     class RedisVersionCheck < SystemCheck::BaseCheck
-      MIN_REDIS_VERSION = '2.8.0'
+      MIN_REDIS_VERSION = '3.2.0'
       RECOMMENDED_REDIS_VERSION = '4.0.0'
       set_name "Redis version >= #{RECOMMENDED_REDIS_VERSION}?"