Replace deprecated ember-cli/ext/promise with rsvp

[leizhao4]

What Changed & Why

• Replaced deprecated ember-cli/ext/promise with rsvp. Ember CLI 2.12 Beta complains about this deprecation.
• Bumped Node version in CI configs to 4 because 0.12 was not happy with consts.

[brunowego]

@leizhao4 thanks for this. Very annoying these warnings on console.

[lukemelia]

@leizhao4 Thanks! Would you mind squashing the 1st and 3rd commits together?

[lukemelia]

Also, I think we should hold off on breaking node 0.12 compatibility, because this addon should still work with older versions of ember-cli, which support 0.12.

[leizhao4]

@lukemelia Sure! I just removed the 2nd commit and squashed the 1st and 3rd. But one dependency username no longer supports Node 0.12 so one test case will fail.

sindresorhus/username#7
stefanpenner/async-disk-cache#43

Apparently this happened three days ago. Now we might need to downgrade async-disk-cache to 1.2.0 in order to maintain 0.12 compatibility.

[leizhao4]

I reported Node 0.12 incompatibility to broccoli-babel-transpiler:

ember-cli/broccoli-babel-transpiler#111

[lukemelia]

@leizhao4 Thanks for your work on this. I incorporated it into a larger change in #36 and will be releasing it as a major version bump.