Testing instructions out of date

[duggelz]

CONTRIB.md says that developers should test changes with make all DEBIAN_SUITE=jessie, but this gives an error message:

cd mkdebootstrap && docker build \
		-t google/mkdebootstrap \
		--build-arg DOCKER_VERSION=1.11.2 \
		.
unable to prepare context: unable to evaluate symlinks in Dockerfile path: lstat /usr/local/google/home/dgreiman/code/github/debian-docker/mkdebootstrap/Dockerfile: no such file or directory
make: *** [mkdebootstrap] Error 1

I think the instructions should instead be something like ./build.sh -r 'gcr.io/cloud-python-runtime-qa' -v jessie except using some other project name. And the Makefile should be updated or removed.

[sharifelgamal]

I think the goal will be to remove the Makefile completely. Just need to change how the travis tests are run and update the README.