Support redis-client gem

[dzunk]

The new https://github.com/redis-rb/redis-client gem is a much simpler and lighter-weight implementation of a Ruby redis client, and drops support for older versions. This module already relies heavily on the low-level .call interface of the redis gem, because all the time-series commands are custom anyway.

Refactoring the gem to support multiple pluggable Redis clients will improve the architecture and leave the client choice in the hands of the end-user.