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.
Also, you made a call to `instances` (not `@instances`) above with your reject which probably recursively called this function.  The return made it not be a problem, but something to be aware of.