Don't set NODE_ENV in the top level of the plugin's index.js

[nerdkid93]

There is a major undocumented side-effect that occurs with importing modernizr-webpack-plugin, namely it sets NODE_ENV to development if it's not already set.

See here for broken line.

This is a major anti-pattern. If NODE_ENV needs to be defined for the plugin to work properly, then it should set it when the plugin is constructed, and even then, it should be documented somewhere that this is happening.