Updating to require pimple ~3.0

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.
Making pimple depend on >=2.1,<4
Merge pull request #39 from andrewshell/patch-2

Updating to require pimple ~3.0