Specify checkout method in CircleCI configs

Checkouts will be blobless by default next month. Getting ahead of that
change by specifying it explicitly.

More info:

https://circleci.com/changelog/introducing-a-faster-checkout-option/

Author: lencioni

.circleci/config.yml

@@ -6,6 +6,8 @@ jobs:
       - image: cimg/node:lts
     working_directory: ~/repo
     steps:
-      - checkout
+      - checkout:
+          method: blobless
+
       - run: yarn install
       - run: yarn test