Add "Latest build" version of Prince XML (#4481)

* Add "Latest build" version of Prince XML

These downloads can be found at https://www.princexml.com/latest/. The download URLs match the same pattern as the regular https://github.com/caskroom/homebrew-cask/blob/master/Casks/prince.rb downloads, so this just needed a different version/sha256.

* prince-latest

* appcast

Casks/prince-latest.rb

@@ -0,0 +1,21 @@
+cask 'prince-latest' do
+  version '20170907'
+  sha256 '3c882cbbcdd646e27263105fce8079391d703c8243ad640b4b2db3364dadce5f'
+
+  url "https://www.princexml.com/download/prince-#{version}-macosx.tar.gz"
+  appcast 'https://www.princexml.com/latest/',
+          checkpoint: '53edcc6bb96a56e42143629a6cc380229ef1f27ed372e550f012e3f71a10f548'
+  name 'Prince'
+  homepage 'https://www.princexml.com/'
+
+  installer script: "prince-#{version}-macosx/install.sh"
+
+  uninstall delete: [
+                      '/usr/local/bin/prince',
+                      '/usr/local/lib/prince',
+                    ]
+
+  caveats do
+    files_in_usr_local
+  end
+end