Updating to require pimple >=2.1,<4

Silex 2.0.x has been updated to require pimple ~3.0 which makes this library incompatible with silex 2.0.x. There is no difference between pimple 2.1.1 and 3.0.0 just a version bump.
Merge pull request #9 from andrewshell/master

Updating to require pimple >=2.1,<4