Fix deploy buddy not saving.

https://zendesk.atlassian.net/browse/SAMSON-117

Skips validation for updating the deploy buddy.

This issue is that the deploy always wants to validate as "deployable".
https://github.com/zendesk/samson/blob/master/app/models/deploy.rb#L11

Either we loosen the validation on the deploy model, or we skip validation when we update the buddy.


`update_attributes` validates, `update_attribute` doesn't.