Update pyyaml to 3.13 (#303)

This PR updates [PyYAML](https://pypi.org/project/PyYAML) from **3.12** to **3.13**.



*The bot wasn't able to find a changelog for this release. [Got an idea?](https://github.com/pyupio/changelogs/issues/new)*

<details>
  <summary>Links</summary>
  
  - PyPI: https://pypi.org/project/pyyaml
  - Homepage: http://pyyaml.org/wiki/PyYAML
</details>

Author: pyup-bot

requirements_dev.txt

@@ -9,6 +9,6 @@ Sphinx==1.7.4
 # only used for setup script
 # cryptography==1.0.1
 py==1.5.3
-PyYAML==3.12
+PyYAML==3.13
 pytest==3.5.1
 prospector[with_everything]==0.12.11