re-revert "ref(tests): switch to example-dockerfile-http app"

example-dockerfile-http has switched over to ubuntu:trusty for better
compatibility with apps:run (busybox does not ship with bash).

This reverts commit ebded177705e257d8227a836bade85331d56f8d7.
fix(builder): delete images on destroy
docs(hacking+README): clarify that deis CLI requires python 2.7
chore(store): remove redundant Makefile task
fix(Makefile): remove CLIENTS from release task

Both deis client and deisctl do not have release tasks.
Merge pull request #2765 from mboersma/deis-cli-py27

docs(hacking+README): clarify that deis CLI requires python 2.7
Merge pull request #2775 from ngpestelos/remove-redundant-makefile-task

chore(store): remove redundant Makefile task
Merge pull request #2747 from bacongobbler/cache-images

fix(builder): delete images on destroy
Merge pull request #2750 from bacongobbler/example-dockerfile-http

re-revert "ref(tests): switch to example-dockerfile-http app"
Merge pull request #2776 from ngpestelos/remove-clients-from-release

fix(Makefile): remove CLIENTS from release task