Loosen Redis dependency with fixed tests

This just combines https://github.com/resque/redis-namespace/pull/81 and https://github.com/resque/redis-namespace/pull/84 in the hopes of getting the Redis dependency relaxed more quickly. (Also #85, though I'm sure #84 will render it unmergeable)

I did none of the work. Thank @adammck and @johanneswuerbach for that. :+1: