Simplify server configuration - remove 'baseDir'

Projects that only use the `baseDir` option can safely promote it to being the value of `server`

```js
server: {
  baseDir: "./app"
}
```

is exactly the same (to BrowserSync) as:

```js
server: "./app"
```

This would not work should you wish to use any *other* server options in the future - but for the current configuration, it does make the gulpfile that little bit cleaner.