sentinel_test.go: cleanup and document that redis 2.8 should be used

As is, with https://packages.debian.org/wheezy/redis-server installed,
we get the following for `$(go test redis)` in `open-nitro/src/redis`:

----------------------------------------------------------------------
FAIL: sentinel_test.go:44: SentinelTestSuite.SetUpSuite

sentinel_test.go:60:
    c.Assert(err, IsNil)
... value *net.OpError = &net.OpError{Op:"dial", Net:"tcp", Addr:(*net.TCPAddr)(0xc2104eb2d0), Err:0x6f} ("dial tcp 127.0.0.1:7379: connection refused")

OOPS: 3 passed, 1 FAILED, 1 MISSED
--- FAIL: Test (2.53 seconds)
FAIL
FAIL    redis   2.541s
test.sh: actually use the redis that we download

Otherwise, on Debian 7 "wheezy" (stable), one has to uninstall
https://packages.debian.org/wheezy/redis-server 2.4.14
for `$(open-nitro/test.sh)` to pass the unit tests
(but then there is no redis for `open-nitro/run.sh`).