npm -> yarn in CI

Author: dmt0

.circleci/config.yml

@@ -14,12 +14,12 @@ jobs:
             - v2-dependencies-{{ checksum "package.json" }}
             - v2-dependencies-
 
-      - run: npm install
+      - run: yarn install
 
       - save_cache:
           paths:
             - node_modules
           key: v2-dependencies-{{ checksum "package.json" }}
 
-      - run: npm test
-      - run: npm run test:percy
+      - run: yarn test
+      - run: yarn test:percy