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Allow ability to retry failed pushes

Since we're testing against live services (heroku & github) sometimes hiccups happen. If a deploy fails unexpectedly, retry it (if set). If it fails every time, then raise an exception.

Add 

```
env:
  global:
  - HATCHET_RETRIES=3
```

To retry each deploy 3 times before failing.
Merge pull request #11 from heroku/schneems/travis-retries

Schneems/travis retries