deps: allow 2.x versions of google-api-core, google-cloud-core, google-resumable-media (#770)

**Note**: PR is empty because this is purely to make sure CHANGELOG is updated. Follow-up to #768

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-bigquery/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #767  🦕

Author: tswast