karma-webpack not generating es5 code

Question

When I run tests in phantom, I get the following error:

PhantomJS 2.1.1 (Mac OS X 0.0.0) ERROR
  SyntaxError: Unexpected token 'const'

When I inspect the test js file that is generated by webpack, it indeed has const declarations despite the configuration trying to target es2015. All tests exist alongside source files, e.g. src/components/comp1.js, src/components/comp1.spec.js

conf/karma.conf.js

const conf = require('./gulp.conf');

module.exports = function (config) {
  const configuration = {
    basePath: '../',
    singleRun: true,
    autoWatch: false,
    logLevel: 'INFO',
    junitReporter: {
      outputDir: 'test-reports'
    },
    browsers: [
      'PhantomJS'
    ],
    frameworks: [
      'jasmine'
    ],
    files: [
      {pattern: 'src/**/*.spec.js', watched: false}
    ],
    preprocessors: {
      'src/**/*.js': ['webpack'],
    },
    reporters: ['progress', 'coverage'],
    coverageReporter: {
      type: 'html',
      dir: 'coverage/'
    },
    webpack: require('./webpack-test.conf'),
    webpackMiddleware: {
      noInfo: false
    },
    plugins: [
      require('karma-jasmine'),
      require('karma-junit-reporter'),
      require('karma-coverage'),
      require('karma-phantomjs-launcher'),
      require('karma-ng-html2js-preprocessor'),
      require('karma-webpack')
    ]
  };

  config.set(configuration);
};

conf/webpack-test.conf.js

module.exports = {
  module: {
    preLoaders: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: 'eslint'
      }
    ],

    loaders: [
      {
        test: /.json$/,
        loaders: [
          'json'
        ]
      },
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loaders: [
          'babel'
        ],
        query: {
          presets: ['es2015']
        }
      },
      {
        test: /\.(css|scss)$/,
        loaders: [
          'style',
          'css',
          'sass',
          'postcss'
        ]
      },
      {
        test: /.html$/,
        loaders: [
          'html'
        ]
      }
    ]
  },
  plugins: [],
  debug: true,
  devtool: 'source-map'
};