Require latest Guzzle

I was getting this error with the current composer.json:

$ php vendor/bin/composer.phar update
Loading composer repositories with package information
Updating dependencies (including require-dev)                
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - guzzlehttp/retry-subscriber 0.1.0 requires guzzlehttp/guzzle 4.0.0-rc.1 -> no matching package found.
    - guzzlehttp/retry-subscriber 0.1.0 requires guzzlehttp/guzzle 4.0.0-rc.1 -> no matching package found.
    - Installation request for guzzlehttp/retry-subscriber 0.1.0 -> satisfiable by guzzlehttp/retry-subscriber[0.1.0].

Potential causes:
 - A typo in the package name
 - The package is not available in a stable-enough version according to your minimum-stability setting
   see <https://groups.google.com/d/topic/composer-dev/_g3ASeIFlrc/discussion> for more details.

Read <http://getcomposer.org/doc/articles/troubleshooting.md> for further common problems.