Define 'call' method on client to enable use with redis-store

The redis-store gem implements a caching strategy for Rails, but swapping in RedisFailover::Client for its default standard Redis client results in errors when redis-store tries to use the "call" method, which it expects to be available, and allows it to programmatically invoke a method, passed as a symbol argument.

So this offered solution defines the "call" method for RedisFailover::Client and delegates it to the already existing "dispatch" method.