Update pytest to 3.6.4 (#22)

This PR updates [pytest](https://pypi.org/project/pytest) from **3.6.3** to **3.6.4**.



<details>
  <summary>Changelog</summary>
  
  
   ### 3.6.4
   ```
   =========================

Bug Fixes
---------

- Invoke pytest using ``-mpytest`` so ``sys.path`` does not get polluted by packages installed in ``site-packages``. (`742 &lt;https://github.com/pytest-dev/pytest/issues/742&gt;`_)


Improved Documentation
----------------------

- Use ``smtp_connection`` instead of ``smtp`` in fixtures documentation to avoid possible confusion. (`3592 &lt;https://github.com/pytest-dev/pytest/issues/3592&gt;`_)


Trivial/Internal Changes
------------------------

- Remove obsolete ``__future__`` imports. (`2319 &lt;https://github.com/pytest-dev/pytest/issues/2319&gt;`_)

- Add CITATION to provide information on how to formally cite pytest. (`3402 &lt;https://github.com/pytest-dev/pytest/issues/3402&gt;`_)

- Replace broken type annotations with type comments. (`3635 &lt;https://github.com/pytest-dev/pytest/issues/3635&gt;`_)

- Pin ``pluggy`` to ``&lt;0.8``. (`3727 &lt;https://github.com/pytest-dev/pytest/issues/3727&gt;`_)
   ```
   
  
</details>


 

<details>
  <summary>Links</summary>
  
  - PyPI: https://pypi.org/project/pytest
  - Changelog: https://pyup.io/changelogs/pytest/
  - Repo: https://github.com/pytest-dev/pytest/issues
  - Homepage: http://pytest.org
</details>

requirements.txt

@@ -2,4 +2,4 @@ flake8==3.5.0
 httplib2==0.11.3
 jsonschema==2.6.0
 markdown-generator==0.1.3
-pytest==3.6.3
+pytest==3.6.4