Add caching to circle config

.circleci/config.yml

@@ -5,9 +5,18 @@ jobs:
       - image: circleci/node:latest
     steps:
       - checkout
+      - restore_cache:
+          keys:
+          - v1-dependencies-{{ checksum "package.json" }}
+          # fallback to using the latest cache if no exact match is found
+          - v1-dependencies-
       - run:
           name: Install
-          command: npm install
+          command: yarn install
+      - save_cache:
+          paths:
+            - node_modules
+          key: v1-dependencies-{{ checksum "package.json" }}
       - run:
           name: Check Prettier, ESLint, Flow
-          command: npm run ci-check
+          command: yarn ci-check