Add Capistrano 3 support for deployment notification task

This adds a new Capistrano task that is compatible with Capistrano 3.

Changes:
- Move old Capistrano task to `lib/new_relic/recipes/capistrano_legacy.rb`
- Create `lib/new_relic/recipes/capistrano3.rb` Capistrano task that works with Capistrano 3
  - This is a modified version of the capistrano_legacy task
- Update readme to include information on usage of the Capistrano task.

This should resolve #154