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/connectivity/connectivity.go

@@ -22,7 +22,8 @@
 package connectivity
 
 import (
-	"golang.org/x/net/context"
+	"context"
+
 	"google.golang.org/grpc/grpclog"
 )
 
@@ -51,7 +52,7 @@ func (s State) String() string {
 const (
 	// Idle indicates the ClientConn is idle.
 	Idle State = iota
-	// Connecting indicates the ClienConn is connecting.
+	// Connecting indicates the ClientConn is connecting.
 	Connecting
 	// Ready indicates the ClientConn is ready for work.
 	Ready