Update Node.js and Yarn

- Node.js: use latest LTS version
 - Yarn: use latest version

circle.yml

@@ -2,24 +2,14 @@
 
 machine:
   node:
-    version: 6.11.2
+    version: 6.11.4
   environment:
-    YARN_VERSION: 0.27.5
-    PATH: "${PATH}:${HOME}/.yarn/bin:${HOME}/${CIRCLE_PROJECT_REPONAME}/node_modules/.bin"
+    PATH: "${PATH}:${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
+    - yarn
   cache_directories:
-    - ~/.yarn
     - ~/.cache/yarn
 
 test:

package.json

@@ -62,7 +62,7 @@
     "heroku-postbuild": "yarn run build"
   },
   "engines": {
-    "node": "6.11.2",
-    "yarn": "0.27.5"
+    "node": "6.11.4",
+    "yarn": "1.2.1"
   }
 }