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/github.com/inconshreveable/mousetrap/README.md

@@ -0,0 +1,23 @@
+# mousetrap
+
+mousetrap is a tiny library that answers a single question.
+
+On a Windows machine, was the process invoked by someone double clicking on
+the executable file while browsing in explorer?
+
+### Motivation
+
+Windows developers unfamiliar with command line tools will often "double-click"
+the executable for a tool. Because most CLI tools print the help and then exit
+when invoked without arguments, this is often very frustrating for those users.
+
+mousetrap provides a way to detect these invocations so that you can provide
+more helpful behavior and instructions on how to run the CLI tool. To see what
+this looks like, both from an organizational and a technical perspective, see
+https://inconshreveable.com/09-09-2014/sweat-the-small-stuff/
+
+### The interface
+
+The library exposes a single interface:
+
+    func StartedByExplorer() (bool)