Update .travis.yml

Hyunje Jun · Hyunje Jun · commit ea760083dbea · 2017-10-23T18:27:30.000+09:00
Remove unused Yarn setting and update Node.js versions.
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,5 @@
 language: node_js
 node_js:
-  - "4"
   - "6"
   - "7"
-cache: yarn
+  - "8"
