Bump pyyaml from 3.12 to 5.1 in /ch11/chapter_11_UNDER_40

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] <[email protected]>

Author: dependabot[bot]

ch11/chapter_11_UNDER_40/requirements.txt

@@ -7,7 +7,7 @@ numpy==1.14.0
 pyparsing==2.2.0
 python-dateutil==2.6.1
 pytz==2018.3
-PyYAML==3.12
+PyYAML==5.1
 scikit-learn==0.19.1
 scipy==1.0.0
 six==1.11.0