codeception is not a function

[isbkch]

I've installed codeception using composer: Codeception version 2.1.5
I've installed laravel-elixir-codeception via npm
but adding mix.codeception() into gulpfile.js and running gulp watch gives the following error
TypeError: mix.codeception is not a function

Any idea about the cause ? I'm running Laravel 5.2 on Debian