Update buildspec; Freeze protobuf and pyarrow

[mabunday]

Issue #, if available:

#106

Description of changes:

1. Invoke tests using tox as part of buildspec
2. Freeze protobuf and pyarrow versions to pass integration tests
3. Update tox dependencies
4. Update base docker image dependencies

Testing:

1. Unit/local container tests
2. Beta integration tests with 515193369038.dkr.ecr.us-west-2.amazonaws.com/sagemaker-scikit-learn:0.23-1-cpu-py3-test
3. Gamma integration tests with 515193369038.dkr.ecr.us-west-2.amazonaws.com/sagemaker-scikit-learn:0.23-1-cpu-py3-test

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.