alchemy/zippy: "^0.3" forces minimum PHP to 7.1 #13

The dependency alchemy/zippy: "^0.3" includes doctrine/collections: "~1.0" which slides up to current doctrine/collections version 1.5 which has a hard requirement for PHP 7.1.

As far as I can tell, alchemy/zippy works great for doctrine/collections <1.5.

So, two options going forward:

1. This project just has a hard dependency on PHP 7.1, no support for PHP 5.6
2. We do some composer ju-ju to force this "grandchild" dependency of doctrine/collections to <1.5.