Upgrade to redis.v2.

This PR upgrades `redismq` to use `gopkg.in/redis.v2` client, which has slightly better API and performance.

```
benchmark                     old ns/op     new ns/op     delta
BenchmarkFourCon1k            358506        289836        -19.15%
BenchmarkFourCon4k            527931        522767        -0.98%
BenchmarkFourPub1k            155492        143724        -7.57%
BenchmarkFourPub4k            156171        114447        -26.72%
BenchmarkFourPubFourCon1k     377791        237466        -37.14%
BenchmarkFourPubFourCon4k     616125        611395        -0.77%
BenchmarkSingPubSingCon1k     211092        195290        -7.49%
BenchmarkSingPubSingCon4k     338034        342645        +1.36%
BenchmarkSingleCon1k          191910        166755        -13.11%
BenchmarkSingleCon4k          305657        222370        -27.25%
BenchmarkSingleConMulti1k     10186598      9008015       -11.57%
BenchmarkSinglePub1k          65112         71977         +10.54%
BenchmarkSinglePub4k          111859        91781         -17.95%
```