Handle case of concurrency limit not being set in Semaphore

Avoid crashing in this case, just assume concurrency limit is 1. This shouldn't
happen with normal operation but we've had this problem when jobs that are
concurrency-limited are enqueued from new code and dispatched from old code,
where the job still doesn't have concurrency controls.

Author: rosa

app/models/solid_queue/semaphore.rb

@@ -70,7 +70,7 @@ def expires_at
         end
 
         def limit
-          job.concurrency_limit
+          job.concurrency_limit || 1
         end
     end
   end