Update atom-beta to 1.18.0-beta3 (#3960)

Casks/atom-beta.rb

@@ -1,11 +1,11 @@
 cask 'atom-beta' do
-  version '1.18.0-beta2'
-  sha256 '0134cc094e16d1d39156dd26aff46e99664567656f83a2fb984db7491f9b0dc8'
+  version '1.18.0-beta3'
+  sha256 '85a03a995dc4434cc08542b3c75d45eea7ef67f524b3b6d18a1d2f813aa23cfc'
 
   # github.com/atom/atom was verified as official when first introduced to the cask
   url "https://github.com/atom/atom/releases/download/v#{version}/atom-mac.zip"
   appcast 'https://github.com/atom/atom/releases.atom',
-          checkpoint: '6fac9860be5ba1bbd0d60bcddc16e7badc07360763d8bfdd1e95bfb874b2d43e'
+          checkpoint: '3469a8f21e316edf9a272af3d7dc7c165175d9185f0d82ad5828c79ff510211b'
   name 'Github Atom Beta'
   homepage 'https://atom.io/beta'