Make circleci use yarn

circle.yml

@@ -1,6 +1,27 @@
 machine:
   node:
     version: 6.2.1
+  environment:
+    YARN_VERSION: 0.18.1
+    PATH: "${PATH}:${HOME}/.yarn/bin:${HOME}/${CIRCLE_PROJECT_REPONAME}/node_modules/.bin"
+
+dependencies:
+  pre:
+    - |
+      if [[ ! -e ~/.yarn/bin/yarn || $(yarn --version) != "${YARN_VERSION}" ]]; then
+        echo "Download and install Yarn."
+        curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version $YARN_VERSION
+      else
+        echo "The correct version of Yarn is already installed."
+      fi
+  override:
+    - yarn install
+  cache_directories:
+    - ~/.yarn
+    - ~/.cache/yarn
+
 test:
+  override:
+    - yarn test
   post:
-    - "npm run coveralls"
+    - "yarn coveralls"