Use yarn instead of npm on circleci

Author: davidyaha

circle.yml

@@ -1,6 +1,17 @@
 machine:
   node:
     version: 6.2.1
+  environment:
+    PATH: "${PATH}:${HOME}/${CIRCLE_PROJECT_REPONAME}/node_modules/.bin"
+
+dependencies:
+  override:
+    - yarn
+  cache_directories:
+    - ~/.cache/yarn
+
 test:
+  override:
+    - yarn test
   post:
-    - npm run coverage
+    - npm run coverage