Use Yarn cache for Travis (facebook#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"