Update electron-beta to 1.6.6 (#3604)

Casks/electron-beta.rb

@@ -1,11 +1,11 @@
 cask 'electron-beta' do
-  version '1.6.5'
-  sha256 'ad2d76b03ecb92cea828968e63c5c93042becd1adfcfebc39db594a7893817d5'
+  version '1.6.6'
+  sha256 'c464773064ff706f02039aafda251f28678943c99bdc5f5a82d553317c14e4c2'
 
   # 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: 'c8f724780b4bc9592dd5708c3fb40fe43fe77fc77dd02ef35d37a715d30e6cdc'
+          checkpoint: 'd0f3af25be0d6fa480d1d7f0801f7f7eaca632521f24c5339c767180b133b7a3'
   name 'Electron'
   homepage 'https://electron.atom.io/'