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

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

db/migrate/20170112151637_create_lti_parameters.rb

@@ -4,7 +4,7 @@ class CreateLtiParameters < ActiveRecord::Migration
       t.belongs_to :external_users
       t.belongs_to :consumers
       t.belongs_to :exercises
-      t.jsonb :lti_parameters, null: false, default: '{}'
+      t.jsonb :lti_parameters, null: false, default: {}
       t.timestamps
     end
   end

db/seeds.rb

@@ -18,7 +18,7 @@ end
 
 # delete all present records
 Rails.application.eager_load!
-(ActiveRecord::Base.descendants - [ActiveRecord::SchemaMigration]).each(&:delete_all)
+(ApplicationRecord.descendants - [ActiveRecord::SchemaMigration]).each(&:delete_all)
 
 # delete file uploads
 FileUtils.rm_rf(Rails.root.join('public', 'uploads'))