Don’t fall-through to calling the callback multiple times.

Prevents the following Webpack error:

````
Error: callback(): The callback was already called.
    at context.callback
(/Users/you/project/node_modules/webpack/node_modules/webpack-core/lib/NormalModuleMixin.js:141:11)
    at Object.module.exports
(/Users/you/project/node_modules/traceur-loader/index.js:57:3)
    at WEBPACK_CORE_LOADER_EXECUTION
(/Users/you/project/node_modules/webpack/node_modules/webpack-core/lib/NormalModuleMixin.js:153:71)
    at runSyncOrAsync
(/Users/you/project/node_modules/webpack/node_modules/webpack-core/lib/NormalModuleMixin.js:153:93)
    at nextLoader
(/Users/you/project/node_modules/webpack/node_modules/webpack-core/lib/NormalModuleMixin.js:266:3)
    at
/Users/you/project/node_modules/webpack/node_modules/webpack-core/lib/NormalModuleMixin.js:268:15
    at runSyncOrAsync
(/Users/you/project/node_modules/webpack/node_modules/webpack-core/lib/NormalModuleMixin.js:158:12)
    at nextLoader
(/Users/you/project/node_modules/webpack/node_modules/webpack-core/lib/NormalModuleMixin.js:266:3)
    at Storage.finished
(/Users/you/project/node_modules/webpack/node_modules/enhanced-resolve/lib/CachedInputFileSystem.js:38:16)
    at fs.js:271:14
```