Setting capacity when initializing a Queue will error out unless RedisQueue has been initialized

@tylermercier 

To fix this error when trying to pass capacity when initializing `Coordinator::Queue`:
```
NoMethodError: undefined method `set' for nil:NilClass
    /home/vagrant/src/coordinator/lib/coordinator/redis_queue.rb:48:in `capacity='
    /home/vagrant/src/coordinator/lib/coordinator/queue.rb:7:in `initialize'
    /home/vagrant/src/coordinator/test/unit/queue_test.rb:11:in `new'
    /home/vagrant/src/coordinator/test/unit/queue_test.rb:11:in `block (3 levels) in <top (required)>'
```

Happens because setting capacity works with Redis, but needs this line to be run before doing so:
https://github.com/tylermercier/coordinator/blob/master/lib/coordinator/redis_queue.rb#L9