v0.8.0 fails to install atom-shell < v0.13.0 due to missing 'arch' in release filename

[banderson]

Background: I'm starting a new project and included the latest version of this plugin. It works fine for installing atom-shell v0.13.0+, but releases prior to that did not include '-arch' suffix in the release filename, and this change causes it to fail.

Repro: running v0.8.0 of this plugin and installing atom-shell v0.12.7 results in this error:
Cannot find atom-shell-v0.12.3-darwin-x64.zip in atom-shell v0.12.3 release

When you look at the github releases, you'll see the actual filename is atom-shell-v0.12.3-darwin.zip.

I switched back to ~0.7.0 to get this working, but I can submit a PR to make this backwards compatible if you'd like.

-Ben

[kevinsawicki]

A PR would be great, thanks for offering 👍