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/golang.org/x/sys/unix/gccgo.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build gccgo
+// +build !aix
 
 package unix