All I could find in Slack's Help is this page, "Using the GitHub app with GitHub Enterprise" which is only for GitHub Enterprise.
Another option that might be useful to you is Zapier's Github-Slack integration: You can push new commits, issues, branches, and other events to Slack as a DM or channel message, or send your Github data to any other supported app.
Disclosure: I work for Zapier
If you're also wanting to get notified on repositories which are not your own (e.g. we use this for new commits to master on Docker containers we use) then you can grab the RSS feed link from the master branch - go to the GH repo, master branch, view source, search for 'atom', copy link).
Add this to the RSS feed Slack app et voila, notification whenever there's a push to master.