Merge pull request #39 from petergoldstein/feature/setup_travis

Add .travis.yml file
Added Travis build status to readme
Update S3 support
Update digitals_controller.rb from master branch

It taken from:

https://github.com/halo/spree_digital/blob/master/app/controllers/spree/digitals_controller.rb
https://github.com/halo/spree_digital/blob/master/spec/controllers/digitals_controller_spec.rb
https://github.com/halo/spree_digital/blob/master/spec/factories/digital_factory.rb
Fix undefined error for rails 3.2
Create other resource url logic