Minor patches.

Some minor patches/updates I noticed when started to debug this gem now.

- [x] Let Travis CI test ruby 2.1.5
- [x] Master branch should use bin not script for Rails 4.x apps.
- [x] Updated badges to point to `spree-contrib`.
- [x] Use document style for spec runs (more verbose).
- [x] Moved contribution information into `CONTRIBUTION.md`
- [x] Update development dependencies.
- [x] Fix deprecated `Spree::Config[:site_url]` with fallback just in case based on Spree version. **I hope ` Spree::Store.current.try(:url)` is correct?** Using try block so we don't need to mock it in specs.

I skipped updating RSpec to 3.x to avoid refactor specs that creates merge conflicts with pending pull request.