Support for silex 2.0-dev

as requested PR to develop branch with addionals
This version constraint is actually really bad… With SemVer you're guaranteed that any 2.x version will have the same API, but you're also guaranteed that 3.x will break backwards compatibility. So you'd want `~2.0` instead, to keep it from matching 3.x (and every other version ever).

(Same goes for the PHPUnit dependency below)