Ensures this addon calls _super in included

[scalvert]

Ember. 3.0.0 removed the use of Ember.WeakMap in favor of native WeakMap. As part of our application's migration to 3.0.0, we found an issue when using that version in conjunction with this addon's use of ember-weakmap.

Specifically, because this addon implements an included hook but does not call _super in that hook, child addons that also implement an included hook don't have their hook invoked.

This is reproducible using the following steps:

1. create a new ember app with the latest version of ember cli - ember new foo --yarn
2. add ember-truncate - yarn add [email protected]
3. run tests - ember t