Allow any 3.7 or 3.8 version of Guzzle

This is the same requirement as what the AWS SDK for PHP uses. It's a
bit like using the pessimistic version operator, but allowing both
"~3.7.0" and "~3.8.0".

This will need to be revisited when Guzzle 3.9 is released. VersionEye
should notify me when that happens.