Update cryptography to 2.1.3

[pyup-bot]

There's a new version of cryptography available.
You are currently using 2.1.1. I have updated it to 2.1.3

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

Changelog

2.1.2

• Corrected a bug with the manylinux1 wheels where OpenSSL's stack was
  marked executable.

.. _v2-1-1:

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

Happy merging! 🤖

[coveralls]

Coverage remained the same at 100.0% when pulling a036202 on pyup-update-cryptography-2.1.1-to-2.1.3 into 55a3604 on master.

[pyup-bot]

Closing this in favor of #59