chore: specify node.js version for travis

.travis.yml

@@ -3,7 +3,7 @@ before_install:
   - curl -o- -L https://yarnpkg.com/install.sh | bash
   - export PATH="$HOME/.yarn/bin:$PATH"
 node_js:
-  - 9
+  - 11
 cache: yarn
 script:
   - yarn lint