Disable webpack cleaning on Linux development

[vitorp]

Done in this PR

When running middleman s on a Linux machine the clean-webpack-plugin cleaning causes middleman to stop serving the assets. This was solved by making plugin param an empty array on Linux. Also this isn't a problem for production builds since middleman doesn't serve those, so i also check if the env is development.

Known issues

• Linux users will need to clean .tmp manually.

To-do

Demo

Please review it, @startae/frontend-team.