Include Rails environment as task dependency (#1205)

Closes #1204

The `suspenders:cleanup:organize_gemfile` task was not loading the Rails
environment, resulting in the following error:

```
undefined method `starts_with?' for an instance of String
```

This was only happening on existing applications not generated by the
[application template][].

[application template]: https://raw.githubusercontent.com/thoughtbot/suspenders/main/lib/install/web.rb

lib/tasks/suspenders.rake

@@ -25,7 +25,7 @@ namespace :suspenders do
 
   namespace :cleanup do
     desc "Organizes Gemfile"
-    task :organize_gemfile do
+    task organize_gemfile: :environment do
       Suspenders::Cleanup::OrganizeGemfile.perform(Rails.root.join("Gemfile"))
     end