Make prebuilt pony-stable available

[SeanTAllen]

Will make it easier for folks to set up their own CI jobs that need stable, if prebuilt stable versions are available.

• available via Bintray for Windows
• available via Bintray for RPM
• available via Bintray for Deb
• available via Homebrew for OSX

Needs #23

[SeanTAllen]

@kulibali I'm looking to get this in place so that people have an easier time installing stable. I'm going to take the release process from ponyc and move it over here as applicable. are you game to move do the same for Appveyor?

[chalcolith]

Sure, can do.

[SeanTAllen]

@kulibali ill let you know when i get started.

[SeanTAllen]

I've opened a PR to get pony-stable into homebrew.

Homebrew/homebrew-core#17962

[SeanTAllen]

Homebrew PR accepted.

[chalcolith]

Once #35 is merged, the AppVeyor build is set up to:

• Grab the latest ponyc build from BinTray and use it to compile pony-stable.
• Name the resulting file pony-stable-{version}-{build}.{git-hash}.zip, where {version} comes from the VERSION file in the root directory.
• For builds on the release branch, deploy to bintray.com/pony-language/pony-stable-win.

[SeanTAllen]

At this point, this is waiting on a fix to ponylang/changelog-tool and then I can move forward with testing a release.