Adding formation section to heroku app.json

[ci skip]
Add more instance build fields to init script context
Implement listing images
Create an ImageFetcherStorer for the API
Synchronise and display the AMI descriptions

These contain timestamps for many of our images, which makes it easier to spot
old ones.
Synchronise and display the AMI state
Fix a typo/syntax error in the image syncing code
Synchronise image name instead of description

We don't use the description, but we do use the name, which is what I meant to
synchronise all along.
Add Slack notifications for Travis CI
Merge pull request #17 from travis-ci/hh-slack-notifications

Add Slack notifications for Travis CI
Auto-merged master into hh-images on deployment.
Error when no images match filter

rather than falling through to a panic condition
Merge pull request #16 from travis-ci/hh-images

Implement listing images
Debug log generated init script
Raise temporary init script + auth expiry, plus make configurable
Use flag that isn't already in use
Add more vars to the whitelist

for better config insight
Copy over the TmpInitExpiry from external to internal config

so that we don't do a SETEX with 0 :scream_cat:
Inject slack username for compat with new rtapi
Include worker count in init script context

so that we can dynamically set the docker storage driver dm.basesize at cloud init time.