remove pinned transformers version

[sugi-chan]

changed the way bert models were loaded in the text and ensemble modules. Not a breaking change

Pull Request Checklist

• [ ] All tests in the tests folder pass with a local build
• [ ] Pull request includes a description of why we are doing this
• [ ] Init files import new capabilities to appropriate level of package (if applicable)
• [x] CHANGELOG has been updated
• [x] Version in _version.py has been updated
• [ ] README has been updated (if applicable)
• [x] requirements.txt and requirements-dev.txt have been recompiled if requirements in setup.py or requirements-dev.in changed.