Modify options passed to broccoli-merge-trees

[notmessenger]

• In order to support this addon being able to be consumed by other addons the option of overwrite: true needs to be passed to broccoli-merge-trees.

• Per the broccoli-merge-trees documentation, the description option does not exist and the annotation option should be used instead. This is true for the whole ^1.1.1 semver range of the dependency.

[notmessenger]

Fails against ember-beta build with

not ok 1 PhantomJS 2.1 - Global error: ReferenceError: Can't find variable: WeakMap at http://localhost:7357/assets/vendor.js, line 33248

@BrianSipple This is the same conversation as https://github.com/BrianSipple/ember-cli-svgstore/pull/20#issuecomment-380143531