Use ^4.0 for drupal-extension

[davereid]

The drupal-extension project finally has alpha tags for the 4.x branch, so it would be good to be a little more stable with the version used. When I attempt to switch my project, it fails:

                + robo update:dependencies
 [Exec] Running composer global require hirak/prestissimo
Changed current directory to /root/.composer
Do not run Composer as root/super user! See https://getcomposer.org/root for details
Using version ^0.3.7 for hirak/prestissimo
./composer.json has been created
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 1 install, 0 updates, 0 removals
  - Installing hirak/prestissimo (0.3.7): Downloading (connecting...)Downloading (0%)           Downloading (15%)Downloading (30%)Downloading (45%)Downloading (65%)Downloading (85%)Downloading (100%)
Writing lock file
Generating autoload files
 [Exec] Done in 5.201s
 [Filesystem\FilesystemStack] remove ["composer.lock"]

 1/1 [============================] 100%
 [Composer\Update] Updating Packages: /usr/local/bin/composer update --optimize-autoloader --ansi

...

  [Composer\DependencyResolver\SolverProblemsException]                                                                                                      
  Problem 1                                                                                                                                                  
      - The requested package drupal/drupal-extension master-dev exists as drupal/drupal-extension[v4.0.0alpha3] but these are rejected by your constraint.                                                                                                                                                           

[deviantintegral]

We should remove https://github.com/deviantintegral/drupal_tests/blob/master/hooks/RoboFile.php#L76 in favour of just the composer.json. Would you like to file a PR @davereid ?