Pin conda installation file for 1.2-1

[edwardjkim]

Description of changes:

Pins Python version in Conda. Without pinning the Python version, a rebuild of the image will bump the Python version when conda-latest bumps the Python version.

Also pins versions for all major dependencies. We have had too many issues related to Conda/Python/libraries getting upgraded inadvertently when a CodeBuild build is triggered. This PR pins the versions in requirements.txt to a specific version or puts an upper bound.

Duplicate entries in test-requirements.txt are removed because it is unnecessary have the libraries listed in requirements.txt again in test-requirements.txt.

The versions were inspected with pip freeze:

docker run 246618743249.dkr.ecr.us-west-2.amazonaws.com/sagemaker-xgboost:0.90-2-cpu-py3 sh -c 'python -m pip freeze'

See also #139.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[edmrossi]

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