Add Slack beta

Casks/slack-beta.rb

@@ -0,0 +1,15 @@
+class SlackBeta < Cask
+  version '0.60 (1700)'
+  sha256 '9245059fdad7914ec911eea121bcca5ccf716ea7b2fbeadc707e618692db89a1'
+
+  url 'https://rink.hockeyapp.net/api/2/apps/06bd6493684f65a3b8f47aca92c9006e/app_versions/8?format=zip&avtoken=06be73491158dac7555587b2b79f9897a8b4827d'
+  homepage 'https://slack.com/'
+  license :closed
+
+  app 'Slack.app'
+
+  caveats do
+    puts 'Initial beta version with multiple team support'
+    os_version_only('10.6', '10.7', '10.8', '10.9', '10.10')
+  end
+end