travis: replace Node 7 with 8

Kevin Delisle · Kevin Delisle · commit add6fd77886d · 2017-07-13T12:35:51.000-04:00
diff --git a/.travis.yml b/.travis.yml
@@ -2,7 +2,7 @@ language: node_js
 node_js:
   - "4"
   - "6"
-  - "7"
+  - "8"
 env:
   - CXX=g++-4.8
 addons:
