Fixed migrations for omniauth plugin

Signed-off-by: Jakub Koudelka <koudelka.j@gmail.com>

plugins/redmine_omniauth_client/db/migrate/001_create_group_mappings.rb

-class CreateGroupMappings < ActiveRecord::Migration
+class CreateGroupMappings < ActiveRecord::Migration[5.2]
   def change
     create_table :group_mappings do |t|
       t.string :variable

plugins/redmine_omniauth_client/db/migrate/002_update_users_ssouid.rb

-class UpdateUsersSsouid < ActiveRecord::Migration
+class UpdateUsersSsouid < ActiveRecord::Migration[5.2]
   def self.up
     change_table :users do |t|
       t.column :sso_u_id, :string