Merge pull request #17 from webpack-contrib/ci/yarn

fix CI build

Author: sokra

.travis.yml

@@ -24,7 +24,7 @@ jobs:
       script: npm run travis:$JOB_PART
       after_success: 'bash <(curl -s https://codecov.io/bash)'
     - stage: Webpack canary
-      before_script: npm i --no-save git://github.com/webpack/webpack.git#master
+      before_script: yarn add git://github.com/webpack/webpack.git#master
       script: npm run travis:$JOB_PART
       node_js: 8
       env: WEBPACK_VERSION=canary JOB_PART=test
@@ -36,7 +36,7 @@ before_install:
 before_script:
   - |-
     if [ "$WEBPACK_VERSION" ]; then
-      npm i --no-save webpack@$WEBPACK_VERSION
+      yarn add webpack@$WEBPACK_VERSION
     fi
 script:
   - 'npm run travis:$JOB_PART'