boto 2.3 support

In boto 2.2.2, a call to S3Connection.get_bucket(BUCKET_NAME) results in a GET request, but in boto 2.32 the same call is now a HEAD request, so save on the bandwidth.

These breaks cumulus.
This is the commit that changed the behavior, in case you're interested: https://github.com/boto/boto/commit/ac7b717d607c9f973245896079312512e77c4cc5