Replace gometalinter with GolangCI-Lint

gometalinter is broken @ HEAD, and I looked into why that was. During
that process, I remembered that we took the linting scripts from
skaffold, and found that in skaffold gometalinter was replaced with
GolangCI-Lint:

https://github.com/GoogleContainerTools/skaffold/pull/619

The change made linting in skaffold faster, so I figured instead of
fixing gometalinter it made more sense to remove it and replace it with
GolangCI-Lint for kaniko as well.

pkg/executor/push.go

@@ -87,7 +87,7 @@ func DoPush(image v1.Image, opts *config.KanikoOptions) error {
 
 		// Create a transport to set our user-agent.
 		tr := http.DefaultTransport
-		if opts.SkipTlsVerify {
+		if opts.SkipTLSVerify {
 			tr.(*http.Transport).TLSClientConfig = &tls.Config{
 				InsecureSkipVerify: true,
 			}