Use yarn instead of npm in building

.travis.yml

@@ -6,9 +6,9 @@ node_js:
 before_install:
   - if [[ `npm -v` != 3* ]]; then npm i -g npm@3; fi
 install:
-  - npm install
-  - npm run build
+  - yarn install
+  - yarn run build
 script:
-  - npm test
+  - yarn run test
 after_success:
-  - npm run coverage
+  - yarn run coverage

appveyor.yml

@@ -7,15 +7,16 @@ install:
   # Get the latest stable version of Node.js or io.js
   - ps: Install-Product node $env:nodejs_version
   # install modules
-  - npm install
+  - yarn install
 
 # Post-install test scripts.
 test_script:
   # Output useful info for debugging.
   - node --version
   - npm --version
+  - yarn --version
   # run tests
-  - npm test
+  - yarn run test
 
 # Don't actually build.
 build: off