mkcert-master: add Homebrew Formula with HEAD stanza

homebrew-core removed all head clauses, so this allows testing the
latest master by instead running

brew install --HEAD https://master.mkcert.dev/mkcert-master.rb

mkcert-master.rb

@@ -0,0 +1,17 @@
+# Copyright 2018 The mkcert Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+class MkcertMaster < Formula
+	desc "Simple tool to make locally trusted development certificates"
+	homepage "https://github.com/FiloSottile/mkcert"
+	head "https://github.com/FiloSottile/mkcert.git"
+
+	depends_on "go" => :build
+
+	def install
+		ENV["GOPATH"] = HOMEBREW_CACHE/"go_cache"
+		system "go", "build", "-o", bin/"mkcert"
+		prefix.install_metafiles
+	end
+end