Upgrade composer requirements to the latest doctrine/collections

I was trying to install rocketeer-slack (a php plugin notifying slack channel whenever somebody deploys an app using Rocketeer) and got stuck into broken dependencies.

My latest laravel pack is using latest doctrine/collections, so I had to fork your package and bump the composer requirements.

Since all tests on travis-ci are passing correctly, could you accept my changes? So that I could create another pull request for aforementioned plugin. :)

Btw, new tag is also needed.