test: add Node.js 10, 12 and 13 in the test matrix

Author: darrachequesne

.travis.yml

@@ -1,10 +1,11 @@
 language: node_js
 sudo: false
 node_js:
-  - "6"
   - "8"
-  - "node"
+  - "10"
+  - "12"
+  - "13"
 notifications:
   irc: "irc.freenode.org#socket.io"
 services:
-  - redis-server
+  - redis