Update to cryptography 1.8.1

[JR Conlin]

A security release of openssl broke one of the calls pyca/cryptography#2750, which will prevent http_ece from installing if there's an existing library that uses a later install of cryptography.

Please update the dependency from 1.1.0 to 1.8.1

[Martin Thomson]

JR, PR?

[JR Conlin]

Added.