Fix Gum installer to use a path that's version agnostic

[dhh]

The current Gum installer is using a download path that's bound to the latest version but also a specific version. So that breaks when Gum does a new release. I fixed the immediate issue in fcb3e9c, but we need to fix the installer such that it uses a fully known good path.

[cyommer]

I created a PR for this, but it may be moot now. Feel free to close if appropriate. My change was very small.

#232