docs: def binder_spec_from_github_url(github_url):
      """
      Converts GitHub origin into a Binder spec.
  
      For example:
      git@github.com:SamLau95/nbinteract.git -> SamLau95/nbinteract/master
      https://github.com/Calebs97/riemann_book -> Calebs97/riemann_book/master
      """
      tokens = re.split(r'/|:', github_url.replace('.git', ''))
      # The username and reponame are the last two tokens
      return '{}/{}/master'.format(tokens[-2], tokens[-1])