Fix for 'no writecb in Transform' error.

The clue came from the discussion here:
https://github.com/rvagg/through2/issues/1

Reproducing:

I'm using gulp-notify to log errors:

    return gulp
      .src('__tests__')
      .pipe(plumber())
      .on("error", notifyError("Jest Error"))
      .pipe(jest(packagejson.jest));
      
This simply might have made the error visible. If a test failed, the callback would be invoked twice.