Docs branch cleanup

With the `master` branch being the main Bourbon library, there is no reason to have all those library-specific files here in the website & docs branch (`gh-pages`), too. Things like `bower.json`, `.npmignore`, `Gemfile`, etc are not needed.

This also makes the main `README` in this docs branch (`gh-pages`) specific to the documentation and not the Bourbon library. It was previously hidden in a directory called `gh-docs` and helps get people set up with using Jekyll and have a local working copy of the marketing & docs site.

- Remove an unused copy of Bourbon in the root (which was unused)
  - The current gh-docs [`README`](https://github.com/thoughtbot/bourbon/blob/gh-pages/gh-docs/README.md) specifies a step to install Bourbon in the root directory. I’m unclear why this would be good.
- Remove library-specific files, like `bower.json`, `.npmignore` & `Gemfile`
- Tweak Jekyll’s config to stop excluding now removed files
- Update license year