Update iterm2-beta: zaps and appcast (#4045)

Just bringing this formula in line with the [iTerm2 stable formula](https://github.com/caskroom/homebrew-cask/blob/master/Casks/iterm2.rb).

Casks/iterm2-beta.rb

@@ -1,8 +1,11 @@
 cask 'iterm2-beta' do
+  # note: "2" is not a version number, but an intrinsic part of the product name
   version '3.1.beta.5'
   sha256 '9b16a793b9f7001dfd190bb3318f79622b5bb590f3d423ea4cd5d17256df92ce'
 
   url "https://iterm2.com/downloads/beta/iTerm2-#{version.dots_to_underscores}.zip"
+  appcast 'https://iterm2.com/appcasts/testing3.xml',
+          checkpoint: '8a198b466129d5221834404f8553a6818801415de19e1bfedb34bcba94014824'
   name 'iTerm2'
   homepage 'https://www.iterm2.com/'
 
@@ -11,5 +14,12 @@ cask 'iterm2-beta' do
 
   app 'iTerm.app'
 
-  zap delete: '~/Library/Preferences/com.googlecode.iterm2.plist'
+  zap delete: [
+                '~/Library/Application Support/com.apple.sharedfilelist/com.apple.LSSharedFileList.ApplicationRecentDocuments/com.googlecode.iterm2.sfl',
+                '~/Library/Application Support/iTerm',
+                '~/Library/Application Support/iTerm2',
+                '~/Library/Caches/com.googlecode.iterm2',
+                '~/Library/Preferences/com.googlecode.iterm2.plist',
+                '~/Library/Saved Application State/com.googlecode.iterm2.savedState',
+              ]
 end