Bump :gem: version to 20.
Merge branch 'master' of github.com:github/pages-gem

* 'master' of github.com:github/pages-gem:
  Update cibuild for new subcommands.
  Create two subcommands: 'versions', 'br'.
  Print directly in executable. h/t @ixti
  Test the executable
  Add executable.
  Output the dependency versions for this build of Pages.