Update Guzzle, PHPUnit and cleanup composer.json

Summary:
Update dependencies like guzzle to version 5
and PHPUnit to Version 4.3.
Also cleaned up the composer.json a little bit by using
the same version format for all dependencies.

Reviewers: #forgetit-dev, claus.due

Reviewed By: #forgetit-dev, claus.due

Differential Revision: https://phabricator.dkd.de/D432