build: upgrade node version for travis

PeachScript · PeachScript · commit 767d25b53200 · 2018-10-26T10:32:46.000+08:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,5 +1,5 @@
 language: node_js
 node_js:
-  - "6"
+  - "8"
 after_success:
   - bash <(curl -s https://codecov.io/bash)
