* Use Github Releases for hosting node binaries
Replaces S3 hosting of node binaries with Github Releases.
`node-pre-gyp publish` works exclusively with S3, so upload step
is now performed by the Travis deployment provider.
The behaviour for the package user should not change.
When building a new version tag, Travis will create a release for the
tag if it does not already exist.
* Switch to the osrm-release-automation machine account rather than personal credentials.
Co-authored-by: Daniel Patterson <danpat@danpat.net>
