Just wanted to confirm the issue stated by @kirkstrobeck. I tried to upgrade the default 1.5.3 install. As stated here, it seems that the Ruby version is outdated: it's 2.1.9 on VM stack and 2.3 minimum required. Even after upgrading it to 2.3, i still got constant errors with the eventmachine when trying to sync with Github:
An error occurred while installing eventmachine (22.214.171.124), and Bundler cannot continue.
Make sure that `gem install eventmachine -v '126.96.36.199'` succeeds before bundling.
As for the method you're proposing, it's a non-solution, as there is no upgraded Discourse VM to choose. And there seems no way to make the upgrade manually.