Emit `k.libsonnet` during `init` subcommand

This commit fixes #113, causing us to emit `k.libsonnet` (a collection
of extensions to the k8s API) by updating the dependency on
`ksonnet-lib/ksonnet-gen` and exercising the new signature for
`ksonnet.Emit`, which handles this automatically.

We update the `vendor/` directory and the code that depends on it in the
same commit so that we don't break bisect.