Remove outdated reference to grunt

[jcass77]

It looks like we might have missed a reference to grunt in the README as part of #1636: https://github.com/pydanny/cookiecutter-django/blame/95ee1f159ddaf481192e49235738785d1bfb8427/%7B%7Bcookiecutter.project_slug%7D%7D/README.rst#L91-L92

Not sure what the gulp equivalent should be though - is it as simple as running Mailhog for all deployments?

[sfdye]

Good find. I don't understand either why we need grunt here.

@webyneter Isn't this enough to use mailhog for non-docker setup?
https://cookiecutter-django.readthedocs.io/en/latest/developing-locally.html#mailhog

[browniebroke]

Thanks @jcass77. You can see what Grunt was doing here. It's pretty simple, but not applicable for Docker.

In non-Docker + Gulp setup, we can implement a Gulp equivalent, by defining a task like this one, but starting Mailhog instead of Django, and add it to the default task dependencies. This way, Mailhog would start in the background.

[webyneter]

@sfdye yes, it should, I specifically reviewed the non-Docker Mailhog docs a few months ago.