pl version: examples/requirements.txt is single source of truth
Author: stas00
Pull request: yes

PL git master is unstable:

```
cd examples/text-classification
./run_pl.sh
```
```
   File "run_pl_glue.py", line 12, in <module>
    from lightning_base import BaseTransformer, add_generic_args, generic_train
  File "/mnt/nvme1/code/huggingface/transformers-master/examples/lightning_base.py", line 7, in <module>
    import pytorch_lightning as pl
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/__init__.py", line 76, in <module>
    __import__('pkg_resources').declare_namespace(__name__)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pkg_resources/__init__.py", line 2301, in declare_namespace
    _handle_ns(packageName, path_item)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pkg_resources/__init__.py", line 2234, in _handle_ns
    loader.load_module(packageName)
  File "/mnt/nvme1/code/github/00pytorch/pytorch-lightning/pytorch_lightning/__init__.py", line 56, in <module>
    from pytorch_lightning.core import LightningDataModule, LightningModule
ImportError: cannot import name 'LightningDataModule' from 'pytorch_lightning.core' (/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/core/__init__.py)
```
the scripts will now rely on:

```
grep pytorch-l examples/requirements.txt
```
```
pytorch-lightning==0.8.5
```

whenever the requirement removed by this PR was added (it also helps to add why it was added):
```
# Install newest ptl.
pip install -U git+http://github.com/PyTorchLightning/pytorch-lightning/
```
seems to no longer be needed - at least the code runs to completion.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6309?src=pr&el=h1) Report
> Merging [#6309](https://codecov.io/gh/huggingface/transformers/pull/6309?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/175cd45e13b2e33d1efec9e2ac217cba99f6ae58&el=desc) will **increase** coverage by `0.07%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6309/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6309?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6309      +/-   ##
==========================================
+ Coverage   79.44%   79.52%   +0.07%     
==========================================
  Files         148      148              
  Lines       27193    27193              
==========================================
+ Hits        21604    21625      +21     
+ Misses       5589     5568      -21     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6309?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6309/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `24.53% <0.00%> (-63.20%)` | :arrow_down: |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6309/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `60.56% <0.00%> (-35.22%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6309/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.46% <0.00%> (+1.00%)` | :arrow_up: |
| [src/transformers/tokenization\_dpr.py](https://codecov.io/gh/huggingface/transformers/pull/6309/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZHByLnB5) | `57.65% <0.00%> (+4.50%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6309/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `97.41% <0.00%> (+32.94%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6309?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6309?src=pr&el=footer). Last update [175cd45...6337bdf](https://codecov.io/gh/huggingface/transformers/pull/6309?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


I am strongly in favor.