Predictable gear uuids

https://trello.com/c/bUe7eOL6/290-enable-predictable-gear-uuids-ose
This code will just set the field value in the memory object and not persist it in mongo 
```
application.gear_index = xyz 
```
To persist it, you can either:
```
application.set :gear_index, <new_value>
```
Or 
```
2. Application.where({"_id" => application._id, "gears.uuid" => self.uuid}).update({"$inc" => {"gears.$.gear_index" => -1}})
```