feat(Node.js): Drop support for Node.js 8

Node.js 8 is no longer supported as it reached [end of
life](https://github.com/nodejs/Release#release-schedule) at the end of 2019.

Connects pelias/pelias#837

Author: orangejulius

Diff for: .travis.yml

@@ -3,7 +3,6 @@ dist: xenial
 notifications:
   email: false
 node_js:
-  - 8
   - 10
   - 12
 script: npm run travis

Diff for: package.json

@@ -25,7 +25,7 @@
     "url": "https://github.com/pelias/model/issues"
   },
   "engines": {
-    "node": ">= 8.0.0"
+    "node": ">= 10.0.0"
   },
   "dependencies": {
     "lodash": "^4.6.1",