Updated Symfony deps

I think it is safe to allow more than just the 2.1 version of the Symfony deps as these components won't have any BC breaks.
Merge pull request #31 from fabpot/patch-1

Updated Symfony deps
Fixed implementation for newer version of helper.
Setting Process current working dir.

Process class is not actually aware of current working directroy, so it must be manually set.
Using explicit file paths.