Use yarn

.travis.yml

@@ -12,9 +12,9 @@ matrix:
     - node_js: "lts/*"
       env: LINT=true
 before_script:
-  - 'if [ "$WEBPACK_VERSION" ]; then npm install webpack@$WEBPACK_VERSION; fi'
+  - 'if [ "$WEBPACK_VERSION" ]; then yarn add webpack@$WEBPACK_VERSION; fi'
 script:
-  - 'if [ -n "${LINT-}" ]; then npm run lint ; fi'
-  - 'if [ -z "${LINT-}" ]; then npm run test-ci ; fi'
+  - 'if [ -n "${LINT-}" ]; then yarn lint ; fi'
+  - 'if [ -z "${LINT-}" ]; then yarn test-ci ; fi'
 after_success:
-  - npm run coverage
+  - yarn coverage

appveyor.yml

@@ -3,9 +3,9 @@ environment:
 
 install:
   - ps: Install-Product node $env:nodejs_version
-  - npm install
+  - yarn
 
 test_script:
-  - npm run test-ci
+  - yarn test-ci
 
 build: off

package.json

@@ -39,10 +39,10 @@
     "build": "babel src/ --out-dir lib/",
     "coverage": "nyc report --reporter=json && codecov -f coverage/coverage-final.json",
     "lint": "eslint src test",
-    "preversion": "npm test",
-    "prepublish": "npm run clean && npm run build",
-    "test": "npm run lint && cross-env BABEL_ENV=test npm run build && npm run test-only",
-    "test-ci": "cross-env BABEL_ENV=test npm run build && npm run test-only",
+    "preversion": "yarn test",
+    "prepublish": "yarn clean && yarn build",
+    "test": "yarn lint && cross-env BABEL_ENV=test yarn build && yarn test-only",
+    "test-ci": "cross-env BABEL_ENV=test yarn build && yarn test-only",
     "test-only": "nyc ava"
   },
   "repository": {