fix: Update package.json build script to use yarn instead of npm. (outline#1476)

* fix: Update package.json build script to yarn.

Update package.json build script to use yarn instead of npm to maintain consistency with the rest of scripts. I was running into an issue with the Dockerfile when using nvm with yarn and this fixed the issue.

Author: jonkurtis

package.json

@@ -6,16 +6,16 @@
     "clean": "rimraf dist",
     "build:webpack": "NODE_ENV=production webpack --config webpack.config.prod.js",
     "build:analyze": "NODE_ENV=production webpack --config webpack.config.prod.js --json > stats.json",
-    "build": "npm run clean && npm run build:webpack",
+    "build": "yarn clean && yarn build:webpack",
     "start": "NODE_ENV=production node index.js",
     "dev": "NODE_ENV=development nodemon --watch server index.js",
     "lint": "eslint app server shared",
     "flow": "flow",
     "deploy": "git push heroku master",
-    "heroku-postbuild": "npm run build && npm run sequelize:migrate",
+    "heroku-postbuild": "yarn build && yarn sequelize:migrate",
     "sequelize:create-migration": "sequelize migration:create",
     "sequelize:migrate": "sequelize db:migrate",
-    "test": "npm run test:app && npm run test:server",
+    "test": "yarn test:app && yarn test:server",
     "test:app": "jest",
     "test:server": "jest --config=server/.jestconfig.json --runInBand --forceExit",
     "test:watch": "jest --config=server/.jestconfig.json --runInBand --forceExit --watchAll"
@@ -191,4 +191,4 @@
     "js-yaml": "^3.13.1"
   },
   "version": "0.46.0"
-}
+}