Update mistune to 0.8.4 (#14)

This PR updates [mistune](https://pypi.org/project/mistune) from **0.8.3** to **0.8.4**.



<details>
  <summary>Changelog</summary>
  
  
   ### 0.8.4
   ```
   ~~~~~~~~~~~~~

Released on Oct. 11, 2018

* Support an escaped pipe char in a table cell. `150`_
* Fix ordered and unordered list. `152`_
* Fix spaces between = in HTML tags
* Add max_recursive_depth for list and blockquote.
* Fix fences code block.

.. _`150`: lepture/mistune#150
.. _`152`: lepture/mistune#152
   ```
   
  
</details>


 

<details>
  <summary>Links</summary>
  
  - PyPI: https://pypi.org/project/mistune
  - Changelog: https://pyup.io/changelogs/mistune/
  - Repo: https://github.com/lepture/mistune
</details>

requirements.txt

@@ -1,7 +1,7 @@
 argutil==1.1.9
 jsonschema==2.6.0
 GitPython==2.1.11
-mistune==0.8.3
+mistune==0.8.4
 flake8==3.5.0
 pytest==3.8.2
 coveralls>=1.2.0