Update /variables endpoint to the latest Core version

[MattiSG]

Module weberror.errormiddleware:171 in __call__
>>  app_iter = self.application(environ, sr_checker)
Module webob.dec:131 in __call__
>>  resp = self.call_func(req, *args, **self.kwargs)
Module webob.dec:196 in call_func
>>  return self.func(req, *args, **kwargs)
Module openfisca_web_api.application:37 in set_application_url
>>  return req.get_response(app)
Module webob.request:1316 in send
>>  application, catch_exc_info=False)
Module webob.request:1280 in call_application
>>  app_iter = application(self.environ, start_response)
Module openfisca_web_api.urls:115 in router
>>  return app(req.environ, start_response)
Module webob.dec:131 in __call__
>>  resp = self.call_func(req, *args, **self.kwargs)
Module webob.dec:196 in call_func
>>  return self.func(req, *args, **kwargs)
Module openfisca_web_api.controllers.variables:80 in api1_variables
>>  get_input_variables_and_parameters = model.get_cached_input_variables_and_parameters,
Module openfisca_core.formulas:586 in to_json
>>  for dated_formula in self.dated_formulas
KeyError: 'stop_instant'