Work around incomplete PyPI release of cryptography

The last step of the wheel-building jobs is to test installing and importing the wheels. The Python cryptogrpahy package is pulled in as a dependency. A new version of cryptography was just released on PyPI, but wheels have not yet been uploaded for all platforms. To avoid having to build the lastest version of cryptography from source, and to avoid needing to install headers and development libraries for openssl, temporarily install the previously released version of cryptography (2.5).