Drop Node 8 support.

Our public API around Node version support says:

> To make it easier for us to deliver on our support promise we have
> adopted a policy that the HEAD of our master branches will be compatible
> with all versions of Node.js currently receiving support. For any Ember
> project on the day we drop support for a Node.js version the following
> things will happen:
>
> * We will update the engines key inside of package.json to specify the
>   versions supported by the project.
> * We will update the testing targets to remove the newly-unsupported
>   Node.js version from the list.
> * We will begin accepting code without testing it against the
    newly-unsupported version of Node.js.
>
> For projects with an unspecified release model we will release one last
> version of the project prior to those steps. The version number change
> will reflect SemVer with respect to the APIs of the project and not take
> into consideration removal of Node.js version support. This means that
> the release may show up as a "patch" version release. For example, if
> we're dropping support for Node.js 0.10 and have a project at version
> 1.3.3 we can drop Node.js 0.10 support when we release version 1.3.4.

(from https://blog.emberjs.com/2016/09/07/ember-node-lts-support.html)

.travis.yml

@@ -95,13 +95,11 @@ jobs:
         - yarn test
 
     - name: Node.js Tests
-      node_js: "8"
       script:
         - yarn ember build -prod
         - yarn test:node
 
     - name: Blueprint Tests
-      node_js: "8"
       script:
         - yarn test:blueprints
 

package.json

@@ -146,7 +146,7 @@
     "tslint": "^5.20.1"
   },
   "engines": {
-    "node": "8.* || 10.* || >= 12.*"
+    "node": "10.* || >= 12.*"
   },
   "ember-addon": {
     "after": "ember-cli-legacy-blueprints"