Babel [karma-babel-processor] Not Converting ES6->ES5 for Karma Tests

We have installed karma, which is using mocha and chai for tests. We are trying to integrate babel straight into karma using karma-babel-processor, to do the converting of our ES6 files into ES5 to be run. Using mocha individually works with babel, i.e. a mocha test command, but we try to use karma instead it doesn't work.

karma.conf.js snippet:

frameworks: ['mocha', 'chai'],

// preprocess matching files before serving them to the browser
// available preprocessors: http://ift.tt/1gyw6MG
preprocessors: {
   'src/**/*.js': ['babel'],
   'test/**/*_spec.js': ['babel']
},

"babelPreprocessor": {
    options: {
        presets: ['es2015'],
        sourceMap: 'inline'
    },   
    filename: function(file) {
        return file.originalPath.replace(/\.js$/, '.es5.js');
    },
    sourceFileName: function(file) {
        return file.originalPath;
    }
},

// list of files / patterns to load in the browser
files: [
  'src/**/*.js',
  'test/**/*_spec.js'
],

package.json snippets:

"scripts": {
  "test": "./node_modules/karma/bin/karma start karma.conf.js"
},

"babel": {
  "presets": ["es2015"]
},

"devDependencies": {
  "babel-preset-es2015": "^6.1.18",
  "chai": "^3.4.1",
  "karma": "^0.13.15",
  "karma-babel-preprocessor": "^6.0.1",
  "karma-chai": "^0.1.0",
  "karma-mocha": "^0.2.1",
  "karma-phantomjs-launcher": "^0.2.1",
  "phantomjs": "^1.9.18",
  "redux": "^3.0.4"
}

We get the following error:

PhantomJS 1.9.8 (Mac OS X 0.0.0) ERROR
  ReferenceError: Can't find variable: exports
  at Users/alexgurr/BT/FutureVoice/trunk/Portal/server/src/login.es5.js:3

When we evaluate the JS files being loaded, they haven't been converted to ES5, hence the syntax 'export' is still present.

We don't want to use any other frameworks for conversion, ie. webpack, browserify etc.

Thanks!