assert file was uploaded
test current buildpack behavior
remove unneeded prepare_buildpack method
re-vendor to anvil-cli 0.15.0
use anvil-cli in heroku org
add it descs
default  to buildkit url if buildpack not specified
resolve buildpack from app context
do not write out .anvil
parse slug url from build response
write cache url to home dir
read cache url from home dir
Send empty string is cache not set.
Add option to clear cache
Merge branch 'dev-app-config'