Fix: Enable ZendDeveloperTools in development mode

This PR adds `ZendDeveloperTools` to the list of modules which should be enabled when the app is switched to development mode.

To switch the app into development mode, apparently you need to run

```
$ php public/index.php development enable
```

See https://github.com/zfcampus/zf-development-mode#to-enable-development-mode.

To actually see the toolbar, you still need to copy the distributed configuration file:

```
$ cp ./vendor/zendframework/zend-developer-tools/config/zenddevelopertools.local.php.dist ./config/autoload
```

See https://github.com/zendframework/ZendDeveloperTools#installation.