feat(builder): cache progrium/cedarish image

This will reduce boot time significantly by making the images
for progrium/cedarish (the image that both deis/slugrunner and
deis/slugbuilder rely on) loaded into the docker daemon before the
deis/slug* images are pulled from the registry.
Merge pull request #1047 from deis/cache-cedarish

feat(builder): cache progrium/cedarish image