use `installer :script` in private-internet-access

* hoist primary command out of `postflight` into `installer` artifact
* this Cask did not use the `caskroom_only` idiom

Casks/private-internet-access.rb

@@ -6,9 +6,9 @@ class PrivateInternetAccess < Cask
   homepage 'https://www.privateinternetaccess.com'
   license :unknown
 
+  installer :script => 'Private Internet Access Installer.app/Contents/MacOS/runner.sh'
+
   postflight do
-      system '/usr/bin/sudo', '-E', '--',
-          "#{destination_path}/Private Internet Access Installer.app/Contents/MacOS/runner.sh"
     system '/usr/bin/sudo', '-E', '--',
            '/usr/sbin/chown', '-R', '--', "#{Etc.getpwuid(Process.euid).name}:staff", Pathname.new(File.expand_path('~')).join('.pia_manager')
   end