Fix missing credentials

Fixes #157 

```
gabriel@alpha:~/example-ruby-sinatra$ deis destroy

 !    WARNING: Potentially Destructive Action
 !    This command will destroy: unisex-jerrycan
 !    To proceed, type "unisex-jerrycan" or re-run this command with --confirm=unisex-jerrycan

> unisex-jerrycan
Destroying unisex-jerrycan... Error! "No credentials provided"
```