Build does not complete due to issue with Py-YAML

[mdhedley]

It appears that the build script is being impacted for the pip change documented in this issue:
pypa/pip#5247

PyYAML no longer installs as a result of this change in pip 10, and as a result all builds are failing, and all pull requests fail to build.

[mdhedley]

You can reproduce this error by running build.sh as is. It appears that the default pyyaml package installed by conda is now 5.15 while the google-cloud-dataflow package requires 4.0 > pyyaml < 3.12. Since conda installed pyyaml pip cannot upgrade/downgrade it and as a result the issue errors out.