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/k8s.io/client-go/tools/cache/OWNERS

@@ -0,0 +1,50 @@
+approvers:
+- thockin
+- lavalamp
+- smarterclayton
+- wojtek-t
+- deads2k
+- caesarxuchao
+- liggitt
+- ncdc
+reviewers:
+- thockin
+- lavalamp
+- smarterclayton
+- wojtek-t
+- deads2k
+- brendandburns
+- derekwaynecarr
+- caesarxuchao
+- mikedanese
+- liggitt
+- nikhiljindal
+- erictune
+- davidopp
+- pmorie
+- kargakis
+- janetkuo
+- justinsb
+- eparis
+- soltysh
+- jsafrane
+- dims
+- madhusudancs
+- hongchaodeng
+- krousey
+- markturansky
+- fgrzadkowski
+- xiang90
+- mml
+- ingvagabund
+- resouer
+- jessfraz
+- david-mcmahon
+- mfojtik
+- '249043822'
+- lixiaobing10051267
+- ddysher
+- mqliang
+- feihujiang
+- sdminonne
+- ncdc