Address improper use of recipients field - fixes spree-contrib/spree_email_to_friend#56
Remove unused parameter hide_recipients

hide_recipients parameter is not used. We simplify gem
functionality by removing it. Related tests are also removed.
Remove html from flash message

Fixes spree-contrib/spree_email_to_friend#55
Test 2.1.5
Use bin folder for Rails 4.x
Update README.md etc.

- Move contributing information to own document.
- Fixes badges.
Semantic versioning.
Update dev dependencies.
Fixes deprecated Spree::Config[:site_url]

It use fallback for older Spree versions.
Update to RSpec 3.1.x
Bye bye hash rockets.
Update README.md for 2-4-stable branch.