fix readme

Author: XieBiao

README.md

@@ -6,7 +6,7 @@ Analyzing memory of redis is to find the keys(prefix) which used a lot of memory
 ## Requirements
 
 * PHP 5.4 or later
-* ext-redis >=2.8.0
+* ext-redis >=2.2.8
 * predis/predis ~1.1.0
 
 ## Installation via Composer([packagist](https://packagist.org/packages/hhxsv5/redis-memory-analysis))