Update SDK version minimum and make file download tag to 1.5.15

After running:
drush make --no-core sites/all/modules/awssdk/awssdk.make
I noticed that string casts on CFSimpleXML objects seemed to be causing a segmentation fault.
The problem (in version 1.5.14 and lower) is the function CFSimpleXML::__toString() calls to_string(). That has a cast of (string)$this. That would then call __toString(), that would then call to_string(), and that would call __toString()..... you get the idea.

That line was corrected here:
http://aws.amazon.com/releasenotes/5261285417896036
As per:
https://github.com/amazonwebservices/aws-sdk-for-php/commit/ed94df642ebe5a95c54a98040c5ae0fda3724d17#diff-75bfd03495b1388b77fd52300d469411

Might be useful to update the make file if possible.