ci(Travis): Add node 8, remove v5

Author: nodkz

.travis.yml

@@ -7,9 +7,9 @@ cache:
 notifications:
   email: true
 node_js:
+  - "8"
   - "7"
   - "6"
-  - "5"
   - "4"
 before_install: yarn global add greenkeeper-lockfile@1
 before_script: greenkeeper-lockfile-update