Drop support for Node.js versions 0.10 and 0.12

Some of our dependencies are no longer supporting pre-4.0 versions of
Node.js. As a result, our CI builds are failing on these platforms.

This pull request removes 0.10 and 0.12 from our Travis CI build matrix
and also adds "engines" field to package.json to tell our internal
Jenkins CI to stop testing 0.10 and 0.12 versions too.

.travis.yml

@@ -1,8 +1,7 @@
 sudo: false
 language: node_js
 node_js:
-  - "0.10"
-  - "0.12"
   - "4"
   - "6"
+  - "8"
 

package.json

@@ -19,6 +19,9 @@
     "posttest": "npm run lint",
     "lint": "eslint ."
   },
+  "engines": {
+    "node": ">=4"
+  },
   "license": "MIT",
   "dependencies": {
     "async": "~0.9.0",