Bump pyyaml from 3.11 to 5.1 in /utils/Sync

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.11 to 5.1.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](https://github.com/yaml/pyyaml/compare/3.11...5.1)

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

utils/Sync/requirements.txt

@@ -5,7 +5,7 @@ Pillow==3.3.0
 pymongo==3.3.0
 PyPDF2==1.26.0
 python-frontmatter==0.3.1
-PyYAML==3.11
+PyYAML==5.1
 requests==2.10.0
 six==1.10.0
 smartcrop==0.1