Build failure with NODE_ENV=production

[GreenGremlin]

When building with process.env.NODE_ENV=production, I'm getting an "undefined is not a function" error.

 48% 166/260 build modules{ [Error: TypeError: undefined is not a function
    at ModernizrPlugin.minifySource (.../node_modules/modernizr-webpack-plugin/index.js:58:30)
] originalError: [TypeError: undefined is not a function] }

The error is occurring on the Object.assign in the ModernizrPlugin.prototype.minifySource call.

[GreenGremlin]

Building with ModernizrWebpackPlugin option minify: false works, but I'd rather not do that.

[GreenGremlin]

Renaming webpack.config.js > webpack.config.babel.js fixed this.

[alexpalombaro]

@GreenGremlin I've updated the plugin to no longer require transpiling with babel or Node > 4
Thanks for lodging the issue 👍