Remove conditional version check on AWS SDK

Uh oh! It finally happened-- the `aws-sdk` gem finally rev'd to 1.10.0, which, although is still in 1.x, seemed to have broken a naive `AWS::VERSION` check at: https://github.com/thoughtbot/paperclip/blob/master/lib/paperclip/storage/s3.rb#L103

Since the comparison thinks 1.3 > 1.1 as a string, it breaks loading of paperclip, which was [reported on our forums](https://forums.aws.amazon.com/thread.jspa?threadID=124605&tstart=0) today.

I've gone ahead and fixed the offending code, using a defined? check on the classes instead, which should be much more future-proof and robust.

Tests seem to pass for me.