set aws-sdk as minimum version

This change will set the minimum version of the amazon-sdk-php to 2.4. (Exactly matching flysystem's)

This will allow users to use a newer version of the sdk (any version where 2.4 <= version < 3.0). There should not be any backwards incompatibility.

References: https://getcomposer.org/doc/01-basic-usage.md#next-significant-release-tilde-operator-

Sorry. I hoped I could get away with not testing a one line change. Apparently not.

I fixed it, and both composer update and phpspec runare working