[Rails5] Make GeoMigrationGenerator compatible with Rails 5

Currently there is an explicit check in ee/lib/generators/rails/geo_migration_generator.rb which raises an error if Rails version is 5: https://gitlab.com/gitlab-org/gitlab-ee/blob/v11.1.1-ee/ee/lib/generators/rails/geo_migration_generator.rb#L5-7

Because we are aiming to switch to Rails 5 soon (https://gitlab.com/gitlab-org/gitlab-ce/issues/48991), we need to verify that GeoMigrationGenerator works with Rails 5 and remove this version check.

/cc @nick.thomas - do you think someone from Geo team could take a look at this? Sooner is better, ideally next week?