I've been using Drupal for several years, but only just dipping a toe in with Drush - I've bought an ebook tutorial and working my way through. I've installed Composer, and used it to install Drush. The tutorial book specified a particular build of Drush to install (7.0.0-alpha5). I've navigated to a drupal folder and run 

```
drush st 

```

to get installation info which works fine.
The tutorial explains that you don't have to navigate into the drupal folder to do this, instead using

```
drush --root=PATH-TO-DRUPAL-SITE st

```

however this brings an error: 

```
Fatal error: Call to undefined function conf_path()

```

Googling this error suggests this a problem with the version of Drush and to upgrade.

SO I've navigated into the .composer/vendor/drush/drush folder and run 
 composer global update
but composer says nothing to update, confirming the version as 7.0.0-alpha5 - I assume this is not the latest 7.x version, so do I need something changing in the composer.json file?