Update cifar.py (#935)
Update cifar.py (#935)

With `Python 3.6.5` the following exception will be thrown:

```
  File "/home/lhlmgr/anaconda3/lib/python3.6/site-packages/tensor2tensor/data_generators/cifar.py", line 146, in cifar_generator
    data = cPickle.load(f)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xff in position 0: ordinal not in range(128)
```
Adding the the parameter `encoding='latin1'` fixes this error for `cifar10` and `cifar100`
tensorflow/tensor2tensor,vthorsteinsson/tensor2tensor,vthorsteinsson/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,vthorsteinsson/tensor2tensor,tensorflow/tensor2tensor,vthorsteinsson/tensor2tensor