AWS S3 in rails - how to set the s3_signature_version parameter

I'm trying to set up the Amazon Simple Storage Service for use with rails. I'm getting this error message:

The authorization mechanism you have provided is not supported. Please use AWS4-HMAC-SHA256.

The problem is that I chose the Frankfurt S3 region, and there only the V4 scheme is supported. It's the same error message as in this post, which directs you to the solution here, with instructions how to "set the :s3_signature_version parameter to :v4 when constructing the client". The command is:

s3 = AWS::S3::Client.new(:s3_signature_version => :v4)

My question is, how do I do this? Where do I put this code?