Fix redis version check

After installing redis' latest stable, check_dependencies chokes on it. The problem is that the version number regex also matches the build number. This makes the regex more terse, and checks for nonnull response rather than a specific response.

````
$ redis-server --version
Redis server v=2.8.8 sha=00000000:0 malloc=libc bits=64 build=502b1b82468da48f
$ redis-server --version | grep -o -E "2.6|2.8"
2.8
246
$ redis-server --version | grep -o -E "2\.6|2\.8" | wc -c | awk {'print $1'}
4
````

We could drop the wc/awk to get the version back, but this seemed more universal.