fix install on macOS

[wyattwalter]

This plugin is currently silently failing on macOS, leaving me with a file with the contents:

Not Found

asdf believes the install was successful. Looking at the goss releases, I see that they're appending -alpha to the file name before the platform for both macOS and Windows currently.

PR changes the following:

• the download will fail when curl encounters a 404
• append -alpha to the end of the binary name on darwin and windows platforms when downloading
• run tests on osx in Travis

I'm not sure when or if they'll change the binary name.

[raimon49]

Thank you!　I was not aware that macOS/Windows was now supported.

I'm not sure when or if they'll change the binary name.

I agree, eventually the -alpha may change to -beta, or it may not come with anything.

[raimon49]

I have confirmed that it can be installed in my macOS environment.

Travis, which is linked to this Git repo, is now read only, so I will migrate it to GitHub Actions at a later date. Thanks for your P-R.