Fix for gulp build --type production

Wikia convention is to refer to production environment by `prod` shortcut and to refer to dev environment by `dev` and it's been changed recently in our config:
* https://github.com/Wikia/config/pull/674,
* https://github.com/Wikia/interactive-maps/pull/217
and you'll find this convention followed on our machines in `/etc/environment`.

Unfortunately `npm install` accepts only `--production` flag and it was ["borrowed" to `deploytools-plugins` helper](https://github.com/Wikia/deploytools-plugins/blob/master/wikia/deploytools_plugins/_utils/gulp.py#L14). We don't want to change it in `deploytools-plugins` and we came with this workaround. We plan to clean it up after switching to Mercury project.