Clean up `package.json`

Remove `update_dist_directory` from the `scripts` property as it no
longer works, nor it is needed¹.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

¹ https://github.com/h5bp/html5-boilerplate/commit/d2740f3c6b2330a3f926f29933b88eb2ecae3d04