Use different tag for image removal tests

When removing an image in tests, remove busybox:ubuntu-14.04 instead of
busybox:buildroot-2013.08.1.

This doesn't change the efficacy of our tests at all, but it does let them
run on CircleCI, where there is some weird interaction between the rootfs
of busybox:buildroot-2013.08.1 and the modifications that the CircleCI
folks have made to the ZFS driver.