For some reason the AWS SDK changed from numeric version numbers to a string version in the latest release: https://github.com/amazonwebservices/aws-sdk-for-php/commit/846506794c96....
Not exactly sure how we want to deal with this. One possibility seems to be to detect version number form the XML files that still use a numeric version. I am hoping this is a mistake which I have posted an issue to find out (https://github.com/amazonwebservices/aws-sdk-for-php/pull/30).
For those who just want a fix
If you want to fix locally make the following change https://github.com/boombatower/aws-sdk-for-php/commit/6d8d5ac83ccc15f98b... or checkout the version branch of my fork https://github.com/boombatower/aws-sdk-for-php/tree/version.