[Ember] Switch to NPM automation (#2071)

- NPM dates are more accurate (usually by a day)
- NPM does not contain version 1.0.0 to 2.11.0 (see https://www.npmjs.com/package/ember-source)

products/emberjs.md

@@ -9,8 +9,12 @@ releasePolicyLink: https://emberjs.com/releases/
 changelogTemplate: https://github.com/emberjs/ember.js/releases/tag/v__LATEST__
 activeSupportColumn: true
 releaseDateColumn: true
+
+# NPM is more accurate than git. Version 1.0.0 to 2.11.0 are not on NPM, but 1.x and 2.x cycles are
+# not displayed on this page so it's not a big deal.
 auto:
--   git: https://github.com/emberjs/ember.js.git
+-   npm: ember-source
+
 releases:
 -   releaseCycle: "4"
     support: true