fix env variable that stores redis version

Environment variable *REDIS_VERSION* does not contain proper value:
```bash
% echo $REDIS_VERSION 
Redis
```
when expected string should look like:
```bash
% echo $REDIS_VERSION 
Redis server v=2.8.11 sha=00000000:0 malloc=jemalloc-3.2.0 bits=64 build=61c7634f4beeb6af
```
so here tiny fix. Please review.