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(#148) using LPOS. Alse we can make #143 more effectively using NX option on EXPIRE.

[jeremy]

Too costly to use for runtime feature detection. Need a different approach if you'd like to use newer Redis capabilities.

Simplest approach, really, is to bump the minimum Redis version to something more modern. Redis 5 is ancient now.

[heka1024]

@jeremy
At first, I looked at rails' implementation and thought it wouldn't be a huge burnden. (https://github.com/rails/rails/blob/4cfdcfef8eedb28a4e0f5d12fae80c2aade02a4c/activesupport/lib/active_support/cache/redis_cache_store.rb#L450-L461) But as you said, it might cause some performance issue. Then, how about memoize result of INFO or define_method dynamically? What I thought is like https://github.com/rails/rails/blob/e5124aed3fdcacb05204391e236c4fd60a1e6e74/activesupport/lib/active_support/notifications/instrumenter.rb#L218-L226.