Do not use ES6 style class exports for non-classes

As with babel6/es2015, that will now result in: `locales.default.en_US`
vs `locales.en_US` /cc @arturi

src/locales/en_US.js

@@ -31,4 +31,4 @@ if (typeof window !== 'undefined' && typeof window.Uppy !== 'undefined') {
   window.Uppy.locales.en_US = en_US
 }
 
-export default en_US
+module.exports = en_US

src/locales/ru_RU.js

@@ -28,4 +28,4 @@ if (typeof window !== 'undefined' && typeof window.Uppy !== 'undefined') {
   window.Uppy.locales.ru_RU = ru_RU
 }
 
-export default ru_RU
+module.exports = ru_RU