[test] Update travis configuration

Author: darrachequesne

.travis.yml

@@ -1,12 +1,14 @@
-sudo: false
 language: node_js
+sudo: false
 node_js:
-  - "4"
-  - "6"
-  - "7"
-
-git:
-  depth: 1
-
+  - '4'
+  - '6'
+  - '8'
+  - node
 notifications:
   irc: "irc.freenode.org#socket.io"
+git:
+  depth: 1
+cache:
+  directories:
+    - node_modules