build(*): update Node from 6 to 8, update Yarn

Angular (2+) switched to Node 8 and so should we.

Closes angular#16360
Ref angular/angular#20807
Ref angular/angular#20832

Author: mgol

.nvmrc

@@ -1 +1 @@
-6
+8

.travis.yml

@@ -1,7 +1,7 @@
 language: node_js
 sudo: false
 node_js:
-  - '6'
+  - '8'
 
 cache:
   yarn: true
@@ -28,7 +28,7 @@ env:
     - secure: oTBjhnOKhs0qDSKTf7fE4f6DYiNDPycvB7qfSF5QRIbJK/LK/J4UtFwetXuXj79HhUZG9qnoT+5e7lPaiaMlpsIKn9ann7ffqFWN1E8TMtpJF+AGigx3djYElwfgf5nEnFUFhwjFzvbfpZNnxVGgX5YbIZpe/WUbHkP4ffU0Wks=
 
 before_install:
-  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 0.27.5
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.3.2
   - export PATH="$HOME/.yarn/bin:$PATH"
 
 before_script:

package.json

@@ -9,8 +9,8 @@
     "url": "https://github.com/angular/angular.js.git"
   },
   "engines": {
-    "node": "^6.9.1",
-    "yarn": ">=0.21.3",
+    "node": "^8.9.1",
+    "yarn": ">=1.3.2",
     "grunt": "^1.2.0"
   },
   "scripts": {