fix: update travis config

.travis.yml

@@ -1,6 +1,9 @@
 language: node_js
+before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash
+  - export PATH="$HOME/.yarn/bin:$PATH"
 node_js:
-  - node
+  - 9
 cache: yarn
 script:
   - yarn lint