build: let Travis build using Node 6, 8 and 10

We give up on Node 4. Also remove it from package.json.

.travis.yml

@@ -1,20 +1,12 @@
 language: node_js
 node_js:
-  - "4"
-  - "6"
-  - "8"
-env:
-  - CXX=g++-4.8
-addons:
-  apt:
-    sources:
-      - ubuntu-toolchain-r-test
-    packages:
-      - g++-4.8
+  - 6
+  - 8
+  - 10
 before_install:
-  - "sudo apt-get update"
-  - "sudo apt-get install libavahi-compat-libdnssd-dev libjpeg-turbo-progs optipng gifsicle"
-  - "npm install -g grunt-cli bower"
+  - sudo apt-get update
+  - sudo apt-get install libavahi-compat-libdnssd-dev libjpeg-turbo-progs optipng gifsicle
+  - npm install -g grunt-cli bower
 script:
-  - "npm test"
-  - "grunt dist"
+  - npm test
+  - grunt dist

package.json

@@ -88,6 +88,6 @@
     "time-grunt": "~1.2.1"
   },
   "engines": {
-    "node": ">=4.8 <11"
+    "node": ">=6 <11"
   }
 }