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/golang.org/x/net/http/httpguts/guts.go

@@ -14,21 +14,6 @@ import (
 	"strings"
 )
 
-// SniffedContentType reports whether ct is a Content-Type that is known
-// to cause client-side content sniffing.
-//
-// This provides just a partial implementation of mime.ParseMediaType
-// with the assumption that the Content-Type is not attacker controlled.
-func SniffedContentType(ct string) bool {
-	if i := strings.Index(ct, ";"); i != -1 {
-		ct = ct[:i]
-	}
-	ct = strings.ToLower(strings.TrimSpace(ct))
-	return ct == "text/plain" || ct == "application/octet-stream" ||
-		ct == "application/unknown" || ct == "unknown/unknown" || ct == "*/*" ||
-		!strings.Contains(ct, "/")
-}
-
 // ValidTrailerHeader reports whether name is a valid header field name to appear
 // in trailers.
 // See RFC 7230, Section 4.1.2