Bump pyyaml from 3.12 to 5.1

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.12 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.12...5.1)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -1,7 +1,7 @@
 jinja2==2.10
 pgpubsub
 numpy==1.14.3
-PyYAML==3.12
+PyYAML==5.1
 cryptography==2.2.2
 scikit-learn==0.19.1
 python-geoip==1.2

requirements_unit_tests.txt

@@ -1,7 +1,7 @@
 jinja2==2.10
 pgpubsub
 numpy==1.14.3
-PyYAML==3.12
+PyYAML==5.1
 cryptography==2.2.2
 scikit-learn==0.19.1
 python-geoip==1.2