Fix S3 endpoint

Amazon is now redirecting on requests that don't use bucket-specific endpoint hosts:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<Error>
  <Code>PermanentRedirect</Code>
  <Message>The bucket you are attempting to access must be addressed using the specified endpoint. Please send all future requests to this endpoint.</Message>
  <Bucket>foobar-bottles</Bucket>
  <Endpoint>foobar-bottles.s3.amAzonaws.com</Endpoint>i
  <RequestId>bar</RequestId>
  <HostId>bat</HostId>
</Error>
```

This fixes that.