title: Webpack compiling vue with NODE_ENV=production still results in dev warning
body: <p>I run an extremely simple web pack config below against a vue.js frontend.  The bundle.js is compiled but still produces the 'You are running Vue in development mode' warning.  I have followed the advice specified <a href="https://v2.vuejs.org/v2/guide/deployment.html" rel="nofollow noreferrer">here</a> but it doesn't change anything.</p>
  <p>Any ideas what's going on please?</p>
  <p>This is my webpack.config.js:</p>
  <pre><code>var webpack = require('webpack')
  
  module.exports = {
  entry: './www/src/js/main.js',
  output: {
    path: &quot;./www/static/js&quot;,
    filename: &quot;bundle.js&quot;
  },
  plugins: [
    new webpack.DefinePlugin({
      'process.env': {
        NODE_ENV: '&quot;production&quot;'
    }
  }),
  new webpack.optimize.UglifyJsPlugin({
      compress: {
        warnings: false
      }
  })
  ],
  module: {
    loaders: [
      {
        test: /\.js$/,
        loader: 'babel',
        exclude: /node_modules/
      },
      {
        test: /\.vue$/,
        loader: 'vue'
      }
    ]
  },
  vue: {
    loaders: {
      js: 'babel'
    }
  }
  }
  </code></pre>