Fix Rails version

db/migrate/001_create_toggl_time_entries.rb

@@ -1,4 +1,4 @@
-class CreateTogglTimeEntries < ActiveRecord::Migration
+class CreateTogglTimeEntries < Rails.version < '5.1' ? ActiveRecord::Migration : ActiveRecord::Migration[4.2]
   def change
     create_table :toggl_time_entries do |t|
     end

db/migrate/002_create_api_key_field.rb

@@ -1,4 +1,4 @@
-class CreateApiKeyField < ActiveRecord::Migration
+class CreateApiKeyField < Rails.version < '5.1' ? ActiveRecord::Migration : ActiveRecord::Migration[4.2]
 
   def up
     custom_field = CustomField.new_subclass_instance('UserCustomField', {
@@ -20,4 +20,4 @@ def down
     CustomField.find_by_name('Toggl API Key').destroy
   end
 
-end
+end

db/migrate/003_set_api_key_not_visible.rb

@@ -1,4 +1,4 @@
-class SetApiKeyNotVisible < ActiveRecord::Migration
+class SetApiKeyNotVisible < Rails.version < '5.1' ? ActiveRecord::Migration : ActiveRecord::Migration[4.2]
 
   def up
     custom_field = CustomField.find_by_name('Toggl API Key')

db/migrate/004_create_workspace_field.rb

@@ -1,4 +1,4 @@
-class CreateWorkspaceField < ActiveRecord::Migration
+class CreateWorkspaceField < Rails.version < '5.1' ? ActiveRecord::Migration : ActiveRecord::Migration[4.2]
 
   def up
     custom_field = CustomField.new_subclass_instance('UserCustomField', {