Add latest_version function (#23)
Add latest_version function (#23)

Eventing and Serving define this function in their test scripts.

It could be useful for other projects as well.

References:
- https://github.com/knative/eventing/blob/12b783a7f2e7fbc54d20af56a53af80476e68df2/test/e2e-common.sh#L68-L74
- https://github.com/knative/serving/blob/631927b80dbd80c2ca6dc45b2638781ed805f919/test/e2e-upgrade-tests.sh#L34
knative/hack,knative/hack