Update the vendored test-infra dir (#80)

Updates `test-infra` to comply with knative/serving#2555, while also getting the latest fixes and features.

Bonus: update the Gopkg files so the cleanup workaround in `update-deps.sh` is not necessary anymore.

Gopkg.toml

@@ -2,9 +2,14 @@
 # for detailed Gopkg.toml documentation.
 
 required = [
-  "github.com/knative/test-infra",
+  "github.com/knative/test-infra/scripts",
 ]
 
+[prune]
+   go-tests = true
+   unused-packages = true
+   non-go = true
 
 [[prune.project]]
   name = "github.com/knative/test-infra"
+  non-go = false

hack/update-deps.sh

@@ -33,6 +33,3 @@ rm dummy.go
 
 rm -rf $(find vendor/ -name 'OWNERS')
 rm -rf $(find vendor/ -name '*_test.go')
-
-# Keep the only dir in knative/test-infra we're interested in
-find vendor/github.com/knative/test-infra -mindepth 1 -maxdepth 1 ! -name scripts -exec rm -fr {} \;