Update aws-sdk to latest

Would be great to update the aws-sdk dep. Any reason not to do this?

Why I'm aiming for this now: mapbox-upload is unnecessarily increasing the size of mbstudio because it pulls in a 17 MB aws-sdk install. Recent versions of aws-sdk are now much smaller: around 2.7 MB after https://github.com/aws/aws-sdk-js/issues/233