Fix apm link for atom-beta 1.13.0-beta8 (#3064)

If you have non-beta atom installed, it will currently overwrite the existing apm link.

Casks/atom-beta.rb

@@ -13,7 +13,7 @@ cask 'atom-beta' do
   depends_on macos: '>= :mountain_lion'
 
   app 'Atom Beta.app'
-  binary "#{appdir}/Atom Beta.app/Contents/Resources/app/apm/node_modules/.bin/apm", target: 'apm'
+  binary "#{appdir}/Atom Beta.app/Contents/Resources/app/apm/node_modules/.bin/apm", target: 'apm-beta'
   binary "#{appdir}/Atom Beta.app/Contents/Resources/app/atom.sh", target: 'atom-beta'
 
   postflight do