Regression Resque.enqeue needs configured resque

in resque 1.24.1 Resque.enqueue works without assigning Resque.redis
first. In 2.0 it failes since the queues are not configured.

To fix it check if redis is set before trying to actually return a
queue, and setup if needed.
Merge pull request #897 from sideshowcoder/regression_enqueue_needs_set_redis

Regression Resque.enqeue needs configured resque