priority data for get_objects_for_workstep, limit stub

This implements

- priority data for WorkflowService.get_objects_for_workstep
- adds stub for passing limit
- considerable cleanup and additions to the documentation
- pins required gem versions to patch lines

Return value for the default with_priority = false
```
WorkflowService.get_objects_for_workstep(... with_priority = false ...)
=> [
  "druid:py156ps0477",
  "druid:tt628cb6479",
  "druid:ct021wp7863"
]
```

or with_priority = true

```
WorkflowService.get_objects_for_workstep(... with_priority = true ...)
=>  {
 "druid:py156ps0477" => 100,
 "druid:tt628cb6479" => 0,
 "druid:ct021wp7863" => -100
}
```