Use yarn build command in predeploy script if using yarn

Created by: alexbrazier

This PR just changes the printed output when running yarn build with a homepage set in the package.json to show yarn build instead of npm run build for the predeploy command if using yarn.

This matches the rest of the output, which also displays the yarn commands.

Changes:

    // ...
    "scripts": {
      // ...
      "predeploy": “npm run build",
      "deploy": "gh-pages -d build"
    }

To:

    // ...
    "scripts": {
      // ...
      "predeploy": “yarn build",
      "deploy": "gh-pages -d build"
    }

Tested manually to check that it is working. Screenshot below: