Fix apm link for atom-beta 1.13.0-beta8

[xdissent]

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

If there’s a checkbox you can’t complete for any reason, that's okay, just explain in detail why you weren’t able to do so.

After making all changes to the cask:

• brew cask audit --download {{cask_file}} is error-free.
• brew cask style --fix {{cask_file}} reports no offenses.
• The commit message includes the cask’s name and version.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
• brew cask install {{cask_file}} worked successfully.
• brew cask uninstall {{cask_file}} worked successfully.
• Checked there are no open pull requests for the same cask.
• Checked that the cask was not already refused in closed issues.