👷 change yarn install to npm install

.circleci/config.yml

@@ -1,7 +1,7 @@
 executors:
   node:
     docker:
-      - image: circleci/node:8.17.0
+      - image: circleci/node:11.10.1
 
 aliases:
   restore_cache: &restore_cache
@@ -11,7 +11,7 @@ aliases:
 
   install_node_modules: &install_node_modules
     run:
-      command: yarn install
+      command: npm install
   save_cache: &save_cache
     save_cache:
       key: yarn-cache-netlify-{{ checksum "yarn.lock" }}