change to s3 gem for quicker installation

I still need to test this on travis, but I think it's working. The aws-sdk gem requires nokogiri, which takes a long time to build. This means that even with bundle_cache, our builds are still taking 5 minutes and up, mostly because of the time taken to install bundle_cache. I've changed the code to use the s3 gem, which doesn't need nokogiri and installs quickly, to avoid this.

Most of the functionality is unaffected; however, the s3 gem does not have support for multipart upload, so I've reverted to a single upload for the cache file.

I will test this in our own travis builds shortly, but I thought I'd open the PR now for discussion of the idea.