Make sure the core bundle is > 2.1.5

This bundle relies on PageableManagerInterface, which is only available
on the master branch of the core bundle. Specifying @dev is not enough
to ensure this. It allows me to install dev-master, but does not force
me to do it, and since I have prefer-stable set to true, Composer
installs 2.2.5, which does not have the interface.