Add new Node.js version 10, remove end-of-life versions 0.10, 0.12, 4.

.travis.yml

@@ -1,10 +1,8 @@
 language: node_js
 node_js:
-  - "0.10"
-  - "0.12"
-  - "4"
   - "6"
   - "8"
+  - "10"
 script: "npm run-script test-cover"
 after_script: "npm install coveralls && cat ./coverage/lcov.info | coveralls"
 sudo: false

package.json

@@ -23,7 +23,7 @@
     "example": "cd example && node standalone.js && grunt markdownlint --force && gulp markdownlint"
   },
   "engines": {
-    "node": ">=0.10"
+    "node": ">=6"
   },
   "dependencies": {
     "markdown-it": "8.4.1"