Report an error when dtach is not installed. This was confusing when first trying to run the specs.
Report an error when dtach is not installed. This was confusing when first trying to run the specs.

redis-store/redis-store,basecamp/redis-store,toxaq/redis-store,shiro16/redis-store,dSmart-srl/redis-store,picocandy/redis-store,milesmatthias/redis-store,ccutrer/redis-store