Update electron-beta to 1.8.1 (#4535)

Casks/electron-beta.rb

@@ -1,11 +1,11 @@
 cask 'electron-beta' do
-  version '1.8.0'
-  sha256 'a2ce8023e1cb7925c7d552b11962d91e4ab953127d0758c02de656246f5cf0aa'
+  version '1.8.1'
+  sha256 '399e10b31b47ae1dc382148df6dc420764eef63f724a3a25cd6c72ba705c1f64'
 
   # github.com/electron/electron was verified as official when first introduced to the cask
   url "https://github.com/electron/electron/releases/download/v#{version}/electron-v#{version}-darwin-x64.zip"
   appcast 'https://github.com/electron/electron/releases.atom',
-          checkpoint: '606f57c65d7dda6d81847e25c573840383b26782ed0089aa9a88f89f3a0d5f73'
+          checkpoint: '0c040fe8196d988aeea8c3480da1aab3b62c57c3eee29fbf7a637f3cfa206479'
   name 'Electron'
   homepage 'https://electron.atom.io/'