Introduce virtualbox-extension-pack-beta (#2235)

fix style
remove unwanted comment

Casks/virtualbox-extension-pack-beta.rb

@@ -0,0 +1,22 @@
+cask 'virtualbox-extension-pack-beta' do
+  version '5.1.0_BETA3,108118'
+  sha256 'e24952ab2d67d4ad2126d4d2053363c839892f0c1fe6f12f40d37d09af86c787'
+
+  url "http://download.virtualbox.org/virtualbox/#{version.before_comma}/Oracle_VM_VirtualBox_Extension_Pack-#{version.before_comma}-#{version.after_comma}.vbox-extpack"
+  name 'Oracle VirtualBox Extension Pack'
+  homepage 'https://www.virtualbox.org'
+  license :closed
+
+  depends_on cask: 'virtualbox-beta'
+  container type: :naked
+
+  stage_only true
+
+  postflight do
+    system 'sudo', 'VBoxManage', 'extpack', 'install', '--replace', "#{staged_path}/Oracle_VM_VirtualBox_Extension_Pack-#{version.before_comma}-#{version.after_comma}.vbox-extpack"
+  end
+
+  uninstall_postflight do
+    system 'sudo', 'VBoxManage', 'extpack', 'uninstall', 'Oracle VM VirtualBox Extension Pack'
+  end
+end