Fix uninitialized constant X::ActiveRecord::VERSION

[vollnhals]

When the class that calls multi_tenant has another ActiveRecord class included, then this reference may resolve to the wrong ActiveRecord class. Make sure that does not happen by using ::ActiveRecord.

This happened for example if activerecord-multi-tenant and mobility gem were used in the same project.

NameError (uninitialized constant Mobility::ActiveRecord::VERSION)

class Model < ApplicationRecord
  extend Mobility

  multi_tenant :tenant
end

The mobility gem defines a class named Mobility::ActiveRecord and includes that into the class above.