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/grpc/peer/peer.go

@@ -21,9 +21,9 @@
 package peer
 
 import (
+	"context"
 	"net"
 
-	"golang.org/x/net/context"
 	"google.golang.org/grpc/credentials"
 )