debug chartpress, disable shallow-clone

[minrk]

• run git diff after chartpress to show changes, in case something suspicious is in values.yaml
• pin specific chartpress commit so that chartpress only changes explicitly (there's also a slight possibility of caching being responsible for the recent weird chartpress behavior in Failed to pull image "jupyterhub/k8s-network-tools:d6340ab" #612)
• rerender values.yaml with recent changes that deviated slightly in style to avoid churn when chartpress rerenders values.yaml

closes #612

[minrk]

chartpress is indeed getting the wrong commits for network-tools and singleuser-sample on Travis. The rest are correct. Interestingly enough, it's getting the right commits when I run it locally. Investigating...

[minrk]

This was caused by shallow-cloning done by travis, which resulted in misidentifying the latest commits of images that haven't changed in a bit. Disabling the shallow-clone behavior has fixed the image tagging.