Fixed migrations for omniauth plugin

Version 5.2 may not be accurate - it's the rails version used in redmine:4.2.1 docker image. It may be different with different redmine image.

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