Add `Kredis#redis_version`

[heka1024]

There are many commands supported on only high versions of redis. For example LPOS or NX option in EXPIRE. So, if we can get version of current redis, we can implement some commands more effectively. For example, we can effectively add include? method for List(https://github.com/rails/kredis/pull/148) using LPOS. Alse we can make https://github.com/rails/kredis/pull/143 more effectively using NX option on EXPIRE.