When I added `AsyncTransformer` I keep getting ...
```
Traceur: const/let declarations in a block containing a yield are not yet implemented
    at Transform._transform (/Users/vojta/Code/templating/node_modules/gulp-traceur/index.js:44:23)
    at Transform._read (/Users/vojta/Code/templating/node_modules/readable-stream/lib/_stream_transform.js:179:10)
    at Transform._write (/Users/vojta/Code/templating/node_modules/readable-stream/lib/_stream_transform.js:167:12)
    at doWrite (/Users/vojta/Code/templating/node_modules/readable-stream/lib/_stream_writable.js:227:10)
    at writeOrBuffer (/Users/vojta/Code/templating/node_modules/readable-stream/lib/_stream_writable.js:217:5)
    at Transform.Writable.write (/Users/vojta/Code/templating/node_modules/readable-stream/lib/_stream_writable.js:184:11)
    at Stream.ondata (stream.js:51:26)
    at Stream.EventEmitter.emit (events.js:95:17)
    at queueData (/Users/vojta/Code/templating/node_modules/map-stream/index.js:43:21)
    at next (/Users/vojta/Code/templating/node_modules/map-stream/index.js:71:7)
```