Update airmail-beta to 3.2.6,428,299 (#3791)

Casks/airmail-beta.rb

@@ -1,11 +1,11 @@
 cask 'airmail-beta' do
-  version '3.2.6.426,296'
-  sha256 '00f9f06d25ff99a59070e3faa447806c1988bc64efa43b9ba44948ede798fb6f'
+  version '3.2.6,428,299'
+  sha256 '94d67eae641ea371372df2e3eb796e9e331ee7c0d918dc89357b28cbbfffdb1b'
 
   # hockeyapp.net/api/2/apps/84be85c3331ee1d222fd7f0b59e41b04 was verified as official when first introduced to the cask
   url "https://rink.hockeyapp.net/api/2/apps/84be85c3331ee1d222fd7f0b59e41b04/app_versions/#{version.after_comma}?format=zip&"
   appcast 'https://rink.hockeyapp.net/api/2/apps/84be85c3331ee1d222fd7f0b59e41b04',
-          checkpoint: '2fc89a068921128bb90a19120256564f184b10335b686ee224f5773ca738b07e'
+          checkpoint: 'bf05521bcc81028159f0e472e293760c8d511dfe538cd4ea58d77a38beb5f781'
   name 'Airmail'
   homepage 'http://airmailapp.com/beta/'