build: update package.json script to use yarn (#636)

Fixes #627

Author: Splaktar

material.angular.io/material.angular.io/material.angular.io/material.angular.io/material.angular.io/package.json

@@ -4,15 +4,15 @@
   "license": "MIT",
   "angular-cli": {},
   "scripts": {
-    "start": "npm run build-themes && ng serve",
-    "start-jit": "npm run build-themes && ng serve --aot=false",
+    "start": "yarn build-themes && ng serve",
+    "start-jit": "yarn build-themes && ng serve --aot=false",
     "lint": "tslint -p src/tsconfig.json",
-    "test": "npm run build-themes && ng test",
+    "test": "yarn build-themes && ng test",
     "pree2e": "webdriver-manager update",
     "e2e": "protractor",
     "build-themes": "bash ./tools/build-themes.sh",
-    "prod-build": "npm run build-themes && ng build --aot --prod",
-    "postinstall": "webdriver-manager update --gecko false && npm run build-themes",
+    "prod-build": "yarn build-themes && ng build --aot --prod",
+    "postinstall": "webdriver-manager update --gecko false && yarn build-themes",
     "publish-prod": "bash ./tools/deploy.sh stable prod",
     "publish-dev": "bash ./tools/deploy.sh",
     "publish-beta": "bash ./tools/deploy.sh stable beta"