Fix issue with disk_storage_human for asset without disks

This release fixes an issue that is easy to run into if you
want to see the disk size of a list of assets but one of them
has no disks at all available. Before this would panic but now
it will return "0 B".

3426411 Additional edge case for Issue 50
ded875c Lock bazel to a known good version
aae5a1e Update travis.yaml
2f1ebb7 Readme update