Update to ember latest

[andyhot]

This PR:

• incorporates all work done in https://github.com/locks/ember-localstorage-adapter/pull/223, https://github.com/locks/ember-localstorage-adapter/pull/222
• further upgrades to ember 3.8
• fixes https://github.com/locks/ember-localstorage-adapter/issues/224 by using Object.assign to shallow copy objects
• adds an extra ember-try scenario for ember-lts-3.4

Thx @jacobq, @joestein216, enjoy @locks !

[petenickless]

hey - just wondering about the status of this, is it possible to merge this PR?