Do not remove images that did not exist in the last run

[pdericson]

If an image did not exist in the last run then it may have just been created.

If the image has just been created and we remove it then we might encounter:

#43

There are two outstanding issues related to #43

1. It is stated in Docker rmi with dangling=true unsafe while images are being pulled moby/moby#8926 (comment) that the fix while applicable to docker pull may not fix the issue for docker build
2. The fix for Docker rmi with dangling=true unsafe while images are being pulled moby/moby#8926 has not been back ported to Docker 1.7.1 (the last version of Docker supported on RHEL 6)

While most people do not care about (2), (1) may still be an issue.