Add Tower beta channel (currently v2.2.2)

This commit adds the Tower beta channel to Versions. It is based on
the main `tower.rb` in `homebrew-cask`, with updates for the beta
appcast.

Casks/tower-beta.rb

@@ -0,0 +1,24 @@
+cask :v1 => 'tower-beta' do
+  version '2.2.2-281'
+  sha256 '83fdbb887c394ed0f76bf91c66972d28bc6d36f144c21dfd582ccb09d5e19864'
+
+  # amazonaws.com is the official download host per the vendor homepage
+  url "https://fournova-app-updates.s3.amazonaws.com/apps/tower2-mac/281-3f2b7672/Tower-2-#{version}.zip"
+  appcast 'https://updates.fournova.com/updates/tower2-mac/beta'
+  name 'Tower'
+  homepage 'http://www.git-tower.com/'
+  license :commercial
+
+  app 'Tower.app'
+  binary 'Tower.app/Contents/MacOS/gittower'
+
+  zap :delete => [
+                  '~/Library/Application Support/com.fournova.Tower2',
+                  '~/Library/Caches/com.fournova.Tower2',
+                  '~/Library/Preferences/com.fournova.Tower2.plist',
+                 ]
+
+  caveats do
+    files_in_usr_local
+  end
+end