[BREAKING] Update ember-macro-helpers version, drops Node 6 support

[GreatWizard]

I had conflict versions of browserslist dependency in my ember-cli-postcss pipeline and I wasn"t able to use a query like > .2% and lastest 2 versions... because ember-moment use an old version of ember-macro-helpers which use an old version of ember-weakmap which use an old version of browserslist (v3.x) ... :D

⚠️ The latest ember-macro-helpers needs to run at least on node v8.* ... so I guess if we merge this PR, a major version of ember-moment will come.

Temporary, I fixed it in my project by using an up-to-date version of browserstlist in my package.json but this was a real headache to understand what happened.

• [x] update ember-macro-helpers to latest
• [x] update engine node version to v8.*

• [x] update travisci configuration to run on node v8

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[GreatWizard]

Oops it's a duplicate of #312 ... I just see it

[jasonmit]

v8.0.0 was released, thanks!

[GreatWizard]

Thanks you!