Switch from Bintray to GitHub releases for hosting npm binaries

[rtfeldman]

@avh4 used GitHub releases to host elm-format binaries.

We know this approach works, and it has several benefits over Bintray:

1. Easier publishing workflow
2. One less potential point of failure; we've experienced Bintray outages in the past, and GitHub outages are already elm-package outages
3. It should address Corporate users cannot access dl.bintray.com #223 because anyone whose corporate firewall blocks GitHub is not going to be able to use elm-package even if they can get the Elm binaries installed.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[rtfeldman]

Tracking in #225