Python 3.4 requires PyYAML<=5.2

[jsiirola]

Fixes: N/A

Summary/Motivation:

This works around a problem with the PyYAML distribution where PyPI doesn't recognize the PyYAML 'python_requires' directive from setup.py.

Changes proposed in this PR:

• force PyYAML<=5.2 for Python 3.4 tests

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[coveralls]

Coverage remained the same at 61.055% when pulling 8485b2f on yaml-ver-fix into d37679f on master.