Add Circle CI configuration

circle.yml

@@ -0,0 +1,27 @@
+# https://circleci.com/docs/install-and-use-yarn/
+
+machine:
+  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:
+  pre:
+    - yarn run build
+  override:
+    - yarn test