refactor: set minimum node version to 8.9

BREAKING CHANGE:
@angular/cli, @ngtools/webpack, @ngtools/logger, and @ngtools/json-schema now require NodeJS 8.9+

Author: clydin

.nvmrc

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

package.json

@@ -31,8 +31,8 @@
     "url": "https://github.com/angular/angular-cli.git"
   },
   "engines": {
-    "node": ">= 6.9.0",
-    "npm": ">= 3.0.0"
+    "node": ">= 8.9.0",
+    "npm": ">= 5.5.1"
   },
   "author": "Angular Authors",
   "license": "MIT",

packages/@angular/cli/package.json

@@ -17,8 +17,8 @@
     "url": "https://github.com/angular/angular-cli.git"
   },
   "engines": {
-    "node": ">= 6.9.0",
-    "npm": ">= 3.0.0"
+    "node": ">= 8.9.0",
+    "npm": ">= 5.5.1"
   },
   "author": "Angular Authors",
   "license": "MIT",

packages/@ngtools/json-schema/package.json

@@ -22,8 +22,8 @@
   },
   "homepage": "https://github.com/angular/angular-cli/tree/master/packages/@ngtools/json-schema",
   "engines": {
-    "node": ">= 4.1.0",
-    "npm": ">= 3.0.0"
+    "node": ">= 8.9.0",
+    "npm": ">= 5.5.1"
   },
   "dependencies": {
   },

packages/@ngtools/logger/package.json

@@ -22,8 +22,8 @@
   },
   "homepage": "https://github.com/angular/angular-cli/tree/master/packages/@ngtools/logger",
   "engines": {
-    "node": ">= 6.9.0",
-    "npm": ">= 3.0.0"
+    "node": ">= 8.9.0",
+    "npm": ">= 5.5.1"
   },
   "dependencies": {
     "rxjs": "^5.0.1"

packages/@ngtools/webpack/package.json

@@ -21,8 +21,8 @@
   },
   "homepage": "https://github.com/angular/angular-cli/tree/master/packages/@ngtools/webpack",
   "engines": {
-    "node": ">= 6.9.0",
-    "npm": ">= 3.0.0"
+    "node": ">= 8.9.0",
+    "npm": ">= 5.5.1"
   },
   "dependencies": {
     "tree-kill": "^1.0.0",