Add new tasks to release to RubyGems and GitHub

1. Follow this instruction: https://github.com/octokit/octokit.rb#using-a-netrc-file
   (you can use a personal token instead of your password).
2. Write the next release's draft on GitHub: https://github.com/guard/guard/releases/new
3. Run `rake release:full` (or `rake release:gem` and then `rake release:github`)
4. Drink a beer and congratulate yourself.