Drop node 6; add 12 to CI

Author: vweevers

.travis.yml

@@ -1,8 +1,8 @@
 language: node_js
 
 node_js:
-  - 6
   - 8
   - 10
+  - 12
 
 after_success: npm run coverage

package.json

@@ -50,6 +50,6 @@
     "ttl"
   ],
   "engines": {
-    "node": ">=6"
+    "node": ">=8"
   }
 }