Update cffi to 1.11.4

[pyup-bot]

There's a new version of cffi available.
You are currently using 1.11.3. I have updated it to 1.11.4

These links might come in handy: PyPI | Changelog | Docs

Changelog

1.11.4

=======

• Windows: reverted linking with python3.dll, because
  virtualenv does not make this DLL available to virtual environments
  for now. See Issue 355_. On Windows only, the C extension
  modules created by cffi follow for now the standard naming scheme
  foo.cp36-win32.pyd, to make it clear that they are regular
  CPython modules depending on python36.dll.

.. _Issue 355: https://bitbucket.org/cffi/cffi/issues/355/

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖