I think this scenario is too long and it tells nothing about textarea for composer.json content that is the most important part of this page. 
You can split this step into 2 scenarios or even 2 features. 
For example ``And I should see "Composer as a service"`` have nothing to do with downloading vendor zip, its more like page overview.
Downloading scenario could be more like:

```
Scenario: Download comporesed vendor files 
    Given I am on the homepage 
    When I fill textarea with value 
    """
    {
        "require": {
            "monolog/monolog": "1.2.*"
        }
    }
    """
   And I press "Go" when it's ready 
   Then I should see "Validating composer.json"
   And I should see "Sending to queue"
   And I should see "Starting async job"
   And I should see "./composer update"
   And I should see "Checking vulnerability"
   And I should see "Serving cached vendor.zip"
```