Adds a min version for Recommends: python-cryptography

[benjaoming]

Since #11 we have been installing python3-cryptography automatically through Recommends (a soft dependency), but this also automatically breaks Kolibri on systems where the version is too old

Fixes #30

[benjaoming]

I think this is a "good enough" fix for #30 - the rest is handled in documentation: https://kolibri.readthedocs.io/en/latest/install/raspberry_pi_manual.html#installing-kolibri

[benjaoming]

Tested locally