RFC: chore(ci): switch to yarn (react-boilerplate#1415)

* chore(ci): switch to yarn

* Remove 'yarn version' because: 'Can't answer a question unless a user TTY'

.travis.yml

@@ -7,11 +7,7 @@ node_js:
 
 script:
   - node ./internals/scripts/generate-templates-for-linting
-  - npm run build
-
-install:
-  - npm i -g npm@latest
-  - npm install
+  - yarn run build
 
 before_install:
   - export CHROME_BIN=chromium-browser
@@ -22,8 +18,9 @@ notifications:
   email:
     on_failure: change
 
-after_success: 'npm run coveralls'
+after_success: 'yarn run coveralls'
 
 cache:
+  yarn: true
   directories:
     - node_modules

appveyor.yml

@@ -25,9 +25,8 @@ install:
   - choco install -y googlechrome
   # Install the latest stable version of Node
   - ps: Install-Product node $env:nodejs_version
-  - npm -g install npm
   - set PATH=%APPDATA%\npm;%PATH%
-  - npm install
+  - yarn
 
 # Disable automatic builds
 build: off
@@ -36,16 +35,16 @@ build: off
 test_script:
   # Output debugging info
   - node --version
-  - npm --version
   - node ./internals/scripts/generate-templates-for-linting
   # run build and run tests
-  - npm run build
+  - yarn run build
 
 # Cache node_modules for faster builds
 cache:
+  - "%LOCALAPPDATA%\\Yarn"
   - node_modules -> package.json
 
 # remove, as appveyor doesn't support secure variables on pr builds
 # so `COVERALLS_REPO_TOKEN` cannot be set, without hard-coding in this file
 #on_success:
-#- npm run coveralls
+#- yarn run coveralls