build: add circle ci config file

circle.yml

@@ -0,0 +1,30 @@
+machine:
+  environment:
+    YARN_VERSION: 0.27.5
+    PATH: "${PATH}:${HOME}/.yarn/bin:${HOME}/${CIRCLE_PROJECT_REPONAME}/node_modules/.bin"
+  node:
+    version: 8
+dependencies:
+  pre:
+    - |
+      if [[ ! -e ~/.yarn/bin/yarn || $(yarn --version) != "${YARN_VERSION}" ]]; then
+        curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version $YARN_VERSION
+      fi
+    - nvm install 4
+    - nvm install 6
+
+  cache_directories:
+    - ~/.yarn
+    - ~/.cache/yarn
+    - "node_modules"
+  override:
+    - yarn install --no-progress
+test:
+  override:
+    - nvm alias default 6
+    - yarn run test-travis
+    - nvm alias default 4
+    - yarn run test-travis
+    - nvm alias default 8
+    - yarn run test-travis
+    - yarn run coverage:codecov