use yarn instead of npm on travis

.travis.yml

@@ -28,15 +28,15 @@ before_install:
       export PATH="/tmp/git-lfs:$PATH"
     fi
 install:
-- npm install
-- npm rebuild node-sass
-- npm run generate-icons
+- yarn
+- yarn upgrade node-sass
+- yarn generate-icons
 before_script:
   - git lfs pull
 script:
 - |
-  npm run build
-  npm test
+  yarn build
+  yarn test
   echo '=== starting travis deploy ==='
   ENVS=`env | grep -iE '^(DEBUG|NODE_|ELECTRON_|YARN_|NPM_|CI|CIRCLE|TRAVIS|APPVEYOR_|CSC_|_TOKEN|_KEY|AWS_|STRIP|BUILD_)([A-Z]|_)*=' | sed -n '/^[^\t]/s/=.*//p' | sed '/^$/d' | sed 's/^/-e /g' | tr '\n' ' '`
   if [ "$TRAVIS_OS_NAME" == "linux" ]; then