chore(travis): yarn install --frozen-lockfile

Author: azu

.travis.yml

@@ -1,3 +1,10 @@
 sudo: false
 language: node_js
-node_js: "stable"
+node_js: stable
+before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.3.2
+  - export PATH=$HOME/.yarn/bin:$PATH
+install:
+  - yarn install --frozen-lockfile
+cache:
+  yarn: true