Update Yarn version.

circle.yml

@@ -19,9 +19,6 @@
 machine:
   node:
     version: 6.9.1
-  environment:
-    YARN_VERSION: 0.19.1
-    PATH: "${PATH}:${HOME}/.yarn/bin:${HOME}/${CIRCLE_PROJECT_REPONAME}/node_modules/.bin"
   services:
     - redis
     - memcached
@@ -35,18 +32,13 @@ general:
 # Install your project's language-specific dependencies
 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
     - yarn global add nyc codecov
   override:
-    - yarn install
+    - yarn
   post:
     - echo $KEYFILE > /home/ubuntu/nodejs-docs-samples/key.json
     - node scripts/install
   cache_directories:
-    - ~/.yarn
     - ~/.cache/yarn
     - appengine/bower/node_modules
     - appengine/cloudsql/node_modules