chore: Dropped support of Node.js 4

Some NPM dependencies dropped support of Node.js 4. It is easier to drop it
too, that looking for workarounds to get the testing build running, or claim
the support without actually testing it.

Add testing with Node.js 10.

BREAKING CHANGE: Dropped support of Node.js 4

.travis.yml

@@ -6,9 +6,9 @@ cache:
 notifications:
   email: false
 node_js:
+  - '10'
   - '8'
   - '6'
-  - '4'
 after_success:
   - npm run travis-deploy-once "npm run semantic-release"
 branches:

README.md

@@ -61,6 +61,7 @@ In lieu of a formal styleguide, take care to maintain the existing coding style.
 
 ## Release History
 
+ * 2018-04-27   v3.0.0   Dropped support of Node.js 4
  * 2018-03-05   v2.2.0   Allow generating page titles from file names without directory
  * 2018-03-04   v2.1.0   Add filtering and accessibility to the reports
  * 2018-03-01   v2.0.0   Change the HTML format to look like Koa11y reports

package.json

@@ -23,7 +23,7 @@
     }
   ],
   "engines": {
-    "node": ">= 4"
+    "node": ">= 6"
   },
   "main": "index.js",
   "scripts": {
@@ -37,10 +37,10 @@
     "object.values": "^1.0.4"
   },
   "devDependencies": {
-    "semantic-release": "^12.4.1",
-    "standard": "^11.0.0",
-    "tap": "^11.1.1",
-    "travis-deploy-once": "^4.3.4"
+    "semantic-release": "^15.1.7",
+    "standard": "^11.0.1",
+    "tap": "^11.1.4",
+    "travis-deploy-once": "^4.4.1"
   },
   "keywords": [
     "grunt-tasks",