Upgrades Guzzle to Guzzle 4

Two initiatives are working towards goals that will require Goutte in Drupal 8's composer.json.
One is adding Behat to Drupal 8
The other is re-architecting Drupals' WebTestBase on top of Mink.
Drupal 8 currently uses Guzzle 4.
We'd rather not rely on Guzzle 3 and Guzzle 4 at the same time.
This updates Goutte to use Guzzle 4.
It will require a new 2.0 branch as it bumps php to 5.4.
I'm happy to help maintain this version.