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

.travis.yml

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

package.json

@@ -23,7 +23,7 @@
     "url": "https://github.com/pelias/mock-logger/issues"
   },
   "engines": {
-    "node": ">=8.0.0",
+    "node": ">=10.0.0",
     "npm": ">=1.4.3"
   },
   "devDependencies": {