Fix issue chrisboulton#19 by ensuring RedisException is not redeclared

Author: Matt Heath

lib/Redisent/Redisent.php

@@ -11,8 +11,11 @@
 
 /**
  * Wraps native Redis errors in friendlier PHP exceptions
+ * Only declared if class doesn't already exist to ensure compatibility with php-redis
  */
-class RedisException extends Exception {
+if (! class_exists('RedisException')) {
+    class RedisException extends Exception {
+    }
 }
 
 /**