fix(builder): delete images on destroy
docs(hacking+README): clarify that deis CLI requires python 2.7
chore(store): remove redundant Makefile task
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