ci: Use yarn on CircleCI

Author: tmcw

circle.yml

@@ -1,8 +1,15 @@
 machine:
   node:
     version: 4
+  environment:
+    PATH: "${PATH}:${HOME}/${CIRCLE_PROJECT_REPONAME}/node_modules/.bin"
+dependencies:
+  override:
+    - yarn
+  cache_directories:
+    - ~/.cache/yarn
 test:
   override:
-    - npm run test-ci
+    - yarn test-ci
   post:
     - ./node_modules/.bin/coveralls < coverage/lcov.info