Print a warning to STDERR if Redis version is less than 2.1.3

Prior to v2.1.3, any writes to keys with an expire/TTL set were
completely deleted before the write took place.

lib/redistat/connection.rb

@@ -4,6 +4,7 @@ module Redistat
   module Connection
 
     REQUIRED_SERVER_VERSION = "1.3.10"
+    MIN_EXPIRE_SERVER_VERSION = "2.1.3"
 
     # TODO: Create a ConnectionPool instance object using Sychronize mixin to replace Connection class
 
@@ -67,6 +68,10 @@ module Redistat
 
       def check_redis_version(conn)
         raise RedisServerIsTooOld if conn.info["redis_version"] < REQUIRED_SERVER_VERSION
+        if conn.info["redis_version"] < MIN_EXPIRE_SERVER_VERSION
+          STDOUT.puts "WARNING: You MUST upgrade Redis to v2.1.3 or later " +
+            "if you are using key expiry."
+        end
         conn
       end