fix for webpack 4 and issue with the webpack callback

[koslo]

for webpack 4 the '.default' isnt needed
for older versions is the call requried

[koslo]

Hi.
There is a issue with the webpack callback. It is called for each file. Running in production takes a long time. I think it is doing the other stuff for each file again.
So when you have 10 mjml files and running in dev it takes for example 30s. In production it will take more than 300s...
This is a solution with Promise.all. Easier is with async/await...
Known issue: watch not working on the mjml files.