[cryptography] Updated to last available version (2.3.1)

[goffi-contrib]

This recipe update cryptography to last version.
Note that "pyasn1" is not anymore a dependency and has been replaced by
"asn1crypto".

[AndreMiras]

Please don't change the url string and don't hardcode the version

[goffi-contrib]

@AndreMiras previous URL is not restored

[AndreMiras]

Thanks for the pull request. CI is currently complaining with the following:

�[1m�[31m[ERROR]�[39m�[0m:   virtualenv is needed to install pure-Python modules, but
�[1m�[31m[ERROR]�[39m�[0m:   virtualenv does not support nesting, and you are running
�[1m�[31m[ERROR]�[39m�[0m:   python-for-android in one. Please run p4a outside of a
�[1m�[31m[ERROR]�[39m�[0m:   virtualenv instead.

But that should be fixed pretty soon with #1437
Hopefully we can merge it this week, and I'll pick yours after that.

[AndreMiras]

@goffi-contrib #1437 just got merged. Do you mind rebasing to see if Travis is now happy about it?

[goffi-contrib]

@AndreMiras done :)

[AndreMiras]

Damn it, they all rely on libffi which requires autoconf.
So it looks like that one can be added to just like in #1441 (comment)
I'm sorry this is embarrassing 😕
The CI still needs a lot of improvements, but it will come.
It could be easy to just add autoconf install to the Dockerfile, but I'm thinking of a more powerful solution.

[AndreMiras]

We actually decided adding the system dependencies to the container https://github.com/kivy/python-for-android/pull/1457/files#diff-3254677a7917c6c01f55212f86c57fbfR51
So I'm going to give your PR a try manually now and if it seems OK, I'll see if I can fix the conflict and merge without disturbing you one more time 😄

[AndreMiras]

I removed the initial comment about the system dependency missing because this was covered in #1457
However Travis is still struggling with cffi which is a dependency of cryptography so I prefer to keep it here for now