Update circleci config to use npm

Author: ryanhefner

.circleci/config.yml

@@ -26,18 +26,18 @@ jobs:
           # fallback to using the latest cache if no exact match is found
           - v1-dependencies-
 
-      - run: yarn install
+      - run: npm i
 
       - save_cache:
           paths:
             - node_modules
           key: v1-dependencies-{{ checksum "package.json" }}
 
       - run: ./node_modules/.bin/greenkeeper-lockfile-update
-      - run: yarn build
+      - run: npm run build
 
       # run tests!
-      - run: yarn test
+      - run: npm run test
 
       # send code coverage info to coveralls
       - run: cat ./coverage/lcov.info | ./node_modules/coveralls/bin/coveralls.js