switch requirement to pimple 2
pimple 2 now sees anon. functions as service definitions
Why do Collections extend Pimple?, reverted to previous definition.
Implemented ArrayAccess and fix tests.
Merge pull request #775 from andrewmman/dev/pimple2

As suggested switch to pimple 2, closes #762. I believe the Collection class was updated to extend Pimple to inherit code common between the two to reduce duplicate code. Semantically I realize that's not ideal. It's a good scenario for traits, to be honest, but even traits are a no-go seeing as we are pulling in Pimple via Composer.