Update pyarrow to meet datasets 1.1.2
# What does this PR do?

Update pyarrow to meet the requirement of y'alls awesome datasets 1.1.2 library for running the examples (tested on the finetune_tiny_bart.sh seq2seq example). You can see the colab I tested on here: https://colab.research.google.com/drive/12XSQoFRlpXLEd_tfvwC5wUVYJZUPKFwE?usp=sharing


Fixes #7691


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dimiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests), 
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Issue #7691
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests? 


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.
@sshleifer 
The `datasets` library already has a requirement on `pyarrow>=0.17.1`: https://github.com/huggingface/datasets/blob/master/setup.py#L68, so this line should not be necessary!<|||||>OH whoops, this must be a colab thing since it already has a `pyarrow` version installed, so what I needed to do was run the pip install with the [`--ignore-installed` flag](https://stackoverflow.com/questions/24764549/upgrade-python-packages-from-requirements-txt-using-pip-command).