Upgrade Rails to version 5.2.1 and adopt code & specs where necessary

Signed-off-by: Sebastian Serth <Sebastian.Serth@student.hpi.de>

spec/factories/error_templates.rb

@@ -1,6 +1,6 @@
 FactoryBot.define do
   factory :error_template do
-    execution_environment { nil }
+    association :execution_environment, factory: :ruby
     name { "MyString" }
     signature { "MyString" }
   end

spec/factories/internal_user.rb

@@ -1,6 +1,7 @@
 FactoryBot.define do
   factory :admin, class: InternalUser do
     activated_user
+    association :consumer
     email { 'admin@example.org' }
     generated_user_name
     password { 'admin' }

spec/factories/structured_error_attributes.rb

@@ -1,7 +1,7 @@
 FactoryBot.define do
   factory :structured_error_attribute do
-    structured_error { nil }
-    error_template_attribute { nil }
+    association :structured_error
+    association :error_template_attribute
     value { "MyString" }
   end
 end

spec/factories/structured_errors.rb

@@ -1,6 +1,6 @@
 FactoryBot.define do
   factory :structured_error do
-    error_template { nil }
-    submission { nil }
+    association :error_template
+    association :submission
   end
 end