Change callback for SREM command to 'int'.

Since Redis 2.4, SREM accepts multiple members so the response callback should cast to `int` instead of `bool`.

    redis 127.0.0.1:6379> SADD myset "one" "two" "three" "four"
    (integer) 4
    redis 127.0.0.1:6379> SREM myset "zero"
    (integer) 0
    redis 127.0.0.1:6379> SREM myset "one"
    (integer) 1
    redis 127.0.0.1:6379> SREM myset "two" "three" "four"
    (integer) 3