Symfony 2.0 compatibility

Hey!

This PR does two things:

 * Fix the test suite by aliasing pdepend master as 2.0.0 because phpmd master requires pdepend 2.0.* (maybe simply removing phpmd from the require-dev is a better solution except if you really need to get the master version).
 * Allow to install the latest version of this library with Symfony 2.0 (currently not possible because guzzle requires symfony ~2.1 since 2.8.0. I have executed the testsuite with guzzle 2.7.0 and all tests are green :))