SSH only should return running instances

This is a fix for [this bug](https://github.com/sportngin/opsicle/issues/63)


Rollback Plan
-------------
* To roll back this change, revert the merge with: `git revert -m 1 MERGE_SHA` and perform another deploy.

URLs
----
https://github.com/sportngin/opsicle/issues/63

QA Plan
-------
Create a .opsicle file for a stack with some stopped instances. Run `bundle exec opsicle ssh {environment}` and make sure stopped instances don't show up.
I would change this to a select:

```ruby
@instances = instances.select { |instance| instance[:status].to_s == 'online'}
```

This will avoid getting "pending"/"setup" boxes as well.