text: Title: Batch generator hangs with multithread
  Body: When using alongside the keras `ImageDataGenerator` with `multithreading=True`, the process hangs on `recv()`. Switching the number of workers to 0 (i.e. no multithread) works as expected. 
  
  > Versions:
  imgaug: 0.2.5 (from master 13May18)
  python: 3.5
  keras: 2.1.3
  
  I saw the last merged PR #126 tacking in the same direction, and have confirmed that the installed version has it, but problem persists.
  