Remove deprecation warnings

[BitBrit]

This PR attempts to remove two deprecation warnings caused by:

1. https://deprecations.emberjs.com/v3.x/#toc_ember-polyfills-deprecate-merge
2. https://deprecations.emberjs.com/v3.x/#toc_deprecate-router-events

[backspace]

This addon currently aims to support Ember versions all the way back to 1.13, so a direct replacement doesn’t work because some of these facilities don’t exist in older versions of Ember.

[bonroyage]

Just throwing out an idea because I'm also looking for ways to clear my application of as many deprecation warnings as possible, one of the other addons I use is ember-router-scroll. In this file they account for backward compatibility by using the gte method provided by ember-compatibility-helpers.

Between lines 22-30 they add listeners for the new router events and between lines 119-133 they extend the mixin again to add the legacy event handlers.