Make guzzle version a little more flexible

Guzzle needed to be exactly 3.9.1. Now it is >=3.9.1

Note that this will never result in Guzzle being upgraded
beyond the 3.x series, because the Github repo and Packagist
locations both changed. To upgrade to 4.x or higher will
require changing the package name to guzzlehttp/guzzle.
Merge pull request #77 from fazy/guzzle-version

Make guzzle version a little more flexible