Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

vendor/google.golang.org/appengine/internal/main_common.go

@@ -0,0 +1,7 @@
+package internal
+
+// MainPath stores the file path of the main package. On App Engine Standard
+// using Go version 1.9 and below, this will be unset. On App Engine Flex and
+// App Engine Standard second-gen (Go 1.11 and above), this will be the
+// filepath to package main.
+var MainPath string