Avoid Pimple 2.1 which has some backward compatibility breaks.

The current version constraint allows Pimple 2.1 to be downloaded, which uses namespaces and different class names. Although there is a `class_alias` call which should maintain backward compatibility, it did not work for me, and https://github.com/fabpot/Pimple/issues/127 shows that there are other conditions where it doesn't work.

Upgrading to 2.1 is probably a good idea, but this is simpler for the immediate need.