Clean up Bundler guidelines

* Remove old Bundler recommendation in Rails section, which has been
  superceded by the Bundler section.
* Alphabetize Bundler guidelines.
* Wrap at 80 characters where possible.
* Add binstubs guideline.
Remove binstubs

https://github.com/thoughtbot/guides/pull/68/files#r3317655
Fix URL to `ruby` directive in `Gemfile` docs