chore(yarn): rely on Travis built-in Yarn support, update Yarn in Jenkins

.travis.yml

@@ -4,8 +4,8 @@ node_js:
   - '6'
 
 cache:
+  yarn: true
   directories:
-    - node_modules
     - bower_components
 
 branches:
@@ -35,10 +35,6 @@ addons:
     packages:
       - g++-4.8
 
-before_install:
-  - curl -o- -L https://raw.githubusercontent.com/yarnpkg/yarn/2a0afc73210c7a82082585283e518eeb88ca19ae/scripts/install-latest.sh | bash -s -- --version 0.17.9
-  - export PATH=$HOME/.yarn/bin:$PATH
-
 before_script:
   - du -sh ./node_modules ./bower_components/ || true
   - ./scripts/travis/before_build.sh

package.json

@@ -10,7 +10,7 @@
   },
   "engines": {
     "node": "^6.9.1",
-    "yarn": ">=0.17.9",
+    "yarn": ">=0.21.3",
     "grunt": "^1.2.0"
   },
   "scripts": {

scripts/jenkins/init-node.sh

@@ -8,7 +8,7 @@ nvm install
 
 # clean out and install yarn
 rm -rf ~/.yarn
-curl -o- -L https://raw.githubusercontent.com/yarnpkg/yarn/2a0afc73210c7a82082585283e518eeb88ca19ae/scripts/install-latest.sh | bash -s -- --version 0.17.9
+curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 0.21.3
 export PATH="$HOME/.yarn/bin:$PATH"
 
 # Ensure that we have the local dependencies installed