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

On Travis we now rely on built-in Yarn support and we only cache the Yarn cache,
not node_modules. This creates a more stable environment as we don't install
over previous node_modules state but we still won't download packages from the
internet in the second run for the same yarn.lock as Yarn takes packages from
its local cache if they exist there.

We install a new Yarn verison manually on Jenkins; the location of the install
script changed.

Closes #15851

.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