Addresses #66.
Addresses #67.
* Update test-infra to use the default build/unit test runners, which includes building go code and linting markdown files.
* Remove the integration tests, as they only start Knative Serving, which is irrelevant for this repo.
Bonuses:
* fix unbuildable `grpc-ping-go` sample
* remove test-infra import hack from `Gopack.lock` and `update-deps.sh`
Includes:
* fixes to `library.sh` in order to have some `hack` scripts working again;
* better checking of PRs where presubmit tests are unnecessary;
Bonus: adds the missing eXecution bit to `update-deps.sh`.
* shared scripts from test-infra live in //vendor/github.com/knative/test-infra/scripts;
* created `//hack/update-deps.sh` to properly update deps, including test-infra;
* all bash scripts were updated to use the vendored scripts;
Part of knative/test-infra#30.