closes #142 - first attempt at adding a redis cache adapter. I've added some testing setup that was broken and documented running them via docker-compose.
I mimicked the dalli cache adapter almost entirely, i think the only choices i had to make were is this included in the flipper-redis gem and how to use the setex cmd (ttl) with redis. I chose yes and to marshal objects when storing / retrieving, limited testing on this show the features serializing in and out of redis ok. Happy to change the implementation if you want.
Side note: when testing in the pry debugger, the inspect method on a feature would call the