Can't load the RobertaTokenizer from AutoTokenizer.from_pretrained interface
## 🐛 Bug

<!-- Important information -->

Model I am using (Bert, XLNet....): `AutoModel`

Language I am using the model on (English, Chinese....): English

The problem arise when using:
* [ ] the official example scripts: (give details)
* [x] my own modified scripts: (give details)

I tried to load a downloaded copy of `roberta-base` with `AutoTokenizer` and I get the following error:

```
Model name 'pretrained_models/roberta-base' was not found in model name list (bert-base-uncased, bert-large-uncased, bert-base-cased, bert-large-cased, bert-base-multilingual-uncased, bert-base-multilingual-cased, bert-base-chinese, bert-base-german-cased, bert-large-uncased-whole-word-masking, bert-large-cased-whole-word-masking, bert-large-uncased-whole-word-masking-finetuned-squad, bert-large-cased-whole-word-masking-finetuned-squad, bert-base-cased-finetuned-mrpc). We assumed 'pretrained_models/roberta-base' was a path or url but couldn't find tokenizer filesat this path or url.
```

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details)

## To Reproduce

The compute nodes of the cluster I am working on are air-gapped, so I downloaded the `roberta-base` model weights, config and vocab files likes so

```bash
$ mkdir -p pretrained_models/roberta-base
$ wget https://s3.amazonaws.com/models.huggingface.co/bert/roberta-base-pytorch_model.bin -O pretrained_models/roberta-base/pytorch_model.bin
$ wget https://s3.amazonaws.com/models.huggingface.co/bert/roberta-base-config.json -O pretrained_models/roberta-base/config.json
$ wget https://s3.amazonaws.com/models.huggingface.co/bert/roberta-base-vocab.json -O pretrained_models/roberta-base/vocab.json
$ wget https://s3.amazonaws.com/models.huggingface.co/bert/roberta-base-merges.txt -O pretrained_models/roberta-base/merges.txt
$ ls pretrained_models/roberta-base
$ config.json  merges.txt  pytorch_model.bin  vocab.json
```
Steps to reproduce the behavior:

```python
>>> from pytorch_transformers import AutoTokenizer
>>> AutoTokenizer.from_pretrained('pretrained_models/roberta-base')
Model name 'pretrained_models/roberta-base' was not found in model name list (bert-base-uncased, bert-large-uncased, bert-base-cased, bert-large-cased, bert-base-multilingual-uncased, bert-base-multilingual-cased, bert-base-chinese, bert-base-german-cased, bert-large-uncased-whole-word-masking, bert-large-cased-whole-word-masking, bert-large-uncased-whole-word-masking-finetuned-squad, bert-large-cased-whole-word-masking-finetuned-squad, bert-base-cased-finetuned-mrpc). We assumed 'pretrained_models/roberta-base' was a path or url but couldn't find tokenizer filesat this path or url.
>>> 
```

## Expected behavior

I expect `AutoTokenizer` to return a `RobertaTokenizer` object initialized with the `vocab.json` and `merges.txt` file from `pretrained_models/roberta-base`.

## Environment

* OS: Ubuntu 18.04
* Python version: 3.7.0
* PyTorch version: 1.2.0
* PyTorch Transformers version (or branch): 1.1.0
* Using GPU ? Yes
* Distributed of parallel setup ? No
* Any other relevant information: Compute nodes are air-gapped so I must download the model on a login node.

## Additional context

If I try to simply provide `roberta-base` to `AutoTokenizer`, I get the same issue

```python
>>> from pytorch_transformers import AutoTokenizer
>>> AutoTokenizer.from_pretrained('roberta-base')
Model name 'roberta-base' was not found in model name list (bert-base-uncased, bert-large-uncased, bert-base-cased, bert-large-cased, bert-base-multilingual-uncased, bert-base-multilingual-cased, bert-base-chinese, bert-base-german-cased, bert-large-uncased-whole-word-masking, bert-large-cased-whole-word-masking, bert-large-uncased-whole-word-masking-finetuned-squad, bert-large-cased-whole-word-masking-finetuned-squad, bert-base-cased-finetuned-mrpc). We assumed 'roberta-base' was a path or url but couldn't find tokenizer filesat this path or url.
```

If I rename `pretrained_models/roberta-base/vocab.json` to `pretrained_models/roberta-base/vocab.txt`, then `AutoModel` returns a `BertTokenizer` object

```bash
$ mv pretrained_models/roberta-base/vocab.json pretrained_models/roberta-base/vocab.txt
```

```python
>>> from pytorch_transformers import AutoTokenizer
>>> AutoTokenizer.from_pretrained('pretrained_models/roberta-base')
<pytorch_transformers.tokenization_bert.BertTokenizer object at 0x7f0de6605588>
>>>
```
This should be fixed on master, can you try to install from master?
(clone the repo and `pip install -e .`).<|||||>Ah, that solved it. Great, thanks a lot!

<|||||>@thomwolf, is this fixed in the latest pip release version?<|||||>Yes, it is available in the latest pip release.