[test] Update travis configuration

Author: darrachequesne

.travis.yml

@@ -2,12 +2,25 @@ language: node_js
 sudo: false
 node_js:
   - '4'
-  - 'node'
+  - '6'
+  - '8'
+  - node
 notifications:
-  irc: irc.freenode.org##socket.io
+  irc: 'irc.freenode.org##socket.io'
 git:
   depth: 1
 matrix:
   include:
-  - node_js: 'node'
-    env: BROWSERS=1
+    - node_js: node
+      env: BROWSERS=1
+cache:
+  directories:
+    - node_modules
+env:
+  global:
+    - secure: >-
+        Ea4P/R9UlWzDlHSP5ynmLiD/YgLjecIvCviOcRTle9mV3P1j2k94Ay1LVu1Jw4whlNmWLq2Z/p8M63L92ODPMlarPsuME8HlP4zGr41whFhRbFdda4k3zrHfUhZBlnhY1MVWXTtVm/l7DOzpBrNh+wKecxZB3yyyEaA+PSG3qcQ=
+    - secure: >-
+        JmPf38qx5Rb6K+WYOMwb5YmESkDmVJ6tgggiJIuyRfHsgQVOO7XBwZuspIKGTSFolUIMaqwQe79Kd+Ehs2ZZ/0lUyF2/6xW3FqFnASUusYJcZdfRjypmBFWs6BRdtEORM8HL0dgBx4O4u/e4ZvtygumbPahjQbMDaqN+MvlpjD0=
+    - secure: >-
+        c3pnLhy3VDJqMl16ABA+8vt3I623aNa2wkLceLXb2V1Dc6eiZeulDH2ekwmdVo/r2WwGIKP3Y6B0mq/xP4W0hg4uT+xWh0AmFHclVyM/yp/AqfXrDUv17Vm0vB7OIgp332OiAlK6Dr13YDbWW8iZxmID41O2+2qohLGPn5JMncg=