Support arioch/redis

[ghoneycutt]

https://github.com/arioch/puppet-redis

Our README states that redis is required, though we don't explain how to set that up. For testing we just install the redis package in Vagrant and use that. A better experience would be to recommend the approved arioch/redis module and update our tests to install that module and use it.

• Update README for arioch/redis
• Update Vagrant to use the redis puppet module instead of installing the redis package directly

[ghoneycutt]

Marked wontfix since the dependency on Redis is going away with Sensu version 2.