uninitialized constant Paperclip::Storage::S3::AWS

When trying to set up the Paperclip gem with Amazon S3 storage I encountered the following error:

Reverting the aws-sdk gem to version < 2.0 fixed it for me:

In Gemfile:

gem 'aws-sdk', '< 2.0'

As Trevor Rore from Amazon writes:

NameError: uninitialized constant AWS
If you receive this error, you likely have a dependency on aws-sdk and have updated so that you now have version 2 installed. Version 2 uses a different module name, so it does not define AWS.

Read more here.