make jedis unit tests pass to Redis 2.8.1 (cleaned ver. of #486)

It's a cleaned version of #486.

I modified unit tests to pass at Redis 2.8.1.

To pass unit tests at Redis 2.8.1, I've fixed some issues.

* commented JedisSentinelTest.clear() and add Redis instance for JedisSentinelTest's slave
  * If slave try to promote master itself, New Sentinel forces to demote it
    * JedisSentinelTest.clear() has no effect, and wait infinitely
    * Slave (6380) is not reusable for another unit tests (as master)
  * previously JedisSentinelTest uses Redis instance (port 6380) shared to Sharding unit tests.
* ipv6 applied to Redis 2.8
  * currently localhost and 127.0.0.1 and ::1 is all same
* Makefile: sleep some time for launch each sentinel (workaround to sentinel's issue)
  * issue to sentinel leader vote: https://github.com/antirez/redis/issues/1419
    * sentinel may confused to vote with sentinels launched approximately same time