Use Yarn cache for Travis (#4774)

* support scoped packages for cra --scripts-version option

* cache yarn

* only cache yarn

* also cache .npm

Author: Jack Zhao

.travis.yml

@@ -5,10 +5,9 @@ node_js:
   - 8
   - 10
 cache:
+  yarn: true
   directories:
-  - node_modules
-  - packages/create-react-app/node_modules
-  - packages/react-scripts/node_modules
+  - .npm
 before_install:
   - curl -o- -L https://yarnpkg.com/install.sh | bash
   - export PATH="$HOME/.yarn/bin:$PATH"