Test 2.1.5
Use bin folder for Rails 4.x
Update README.md etc.

- Move contributing information to own document.
- Fixes badges.
Update dev dependencies.

- Use document style for spec runs.
- Include pry in spec for easy debug.
- Keep RSpec 2.14 to avoid spec refactor for now to avoid merge
conflict with pending pull request.
- Gemfile don’t need to declare master branch because it default.
Fixes deprecated Spree::Config[:site_url]

It use fallback for older Spree versions.