fix cryptography dependency

[gargravarr]

In some occasions, whan upgrading the STUPS tools, an error is raised related to Pythons's cryptography library being outdated. Update to its latest version in the requirements.

% stups configure stups.zalan.do
  ___ _____ _   _ ___  ___
 / __|_   _| | | | _ \/ __| by ZALANDO
 \__ \ | | | |_| |  _/\__ \
 |___/ |_|  \___/|_|  |___/
 STUPS - To Unleash Penguin Swarms
 Homepage:      https://stups.io
 Documentation: https://docs.stups.io
 GitHub Repos:  https://github.com/zalando-stups
Please enter your STUPS domain (e.g. "stups.example.org") [stups.zalan.do]:
Trying to autoconfigure mai.. OK
Trying to autoconfigure zign.. OK
Trying to autoconfigure zalando-token-cli.. OK
Trying to autoconfigure zalando-aws-cli.. OK
Checking https://pierone.stups.zalan.do.. OK
Checking https://even.stups.zalan.do.. OK
Checking https://fullstop.stups.zalan.do.. OK
Checking https://kio.stups.zalan.do.. OK
Please enter your OAuth username if it differs from $USER (e.g. "jdoe") [sszuecs]:
Writing global config.. OK
Writing config for Pier One.. OK
Writing config for Più.. OK
Writing config for Fullstop.. OK
Writing config for Kio.. OK
Writing config for Zign.. OK
Writing config for Zalando Token CLI.. OK
Writing config for Zalando AWS CLI.. OK
Now running "mai create-all" to configure your AWS profile(s)..
Please use the following pattern for your SAML username: <firstname>.<lastname>@zalando.<tld>
SAML username: sandor.szuecs@zalando.de
Traceback (most recent call last):
  File "/usr/local/bin/mai", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.4/dist-packages/mai/cli.py", line 322, in main
    cli()
  File "/usr/local/lib/python3.4/dist-packages/click/core.py", line 716, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.4/dist-packages/click/core.py", line 696, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.4/dist-packages/click/core.py", line 1060, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.4/dist-packages/click/core.py", line 889, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.4/dist-packages/click/core.py", line 534, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.4/dist-packages/click/decorators.py", line 27, in new_func
    return f(get_current_context().obj, *args, **kwargs)
  File "/usr/local/lib/python3.4/dist-packages/mai/cli.py", line 144, in create_all
    saml_xml, roles = saml_login(user, url)
  File "/usr/local/lib/python3.4/dist-packages/mai/cli.py", line 201, in saml_login
    saml_password = keyring.get_password('mai', ring_user)
  File "/usr/local/lib/python3.4/dist-packages/keyring/core.py", line 42, in get_password
    return _keyring_backend.get_password(service_name, username)
  File "/usr/local/lib/python3.4/dist-packages/keyring/backends/SecretService.py", line 64, in get_password
    return item.get_secret().decode('utf-8')
  File "/usr/local/lib/python3.4/dist-packages/secretstorage/item.py", line 102, in get_secret
    decryptor = Cipher(aes, modes.CBC(aes_iv), default_backend()).decryptor()
  File "/usr/local/lib/python3.4/dist-packages/cryptography/hazmat/backends/__init__.py", line 71, in default_backend
    _default_backend = MultiBackend(_available_backends())
  File "/usr/local/lib/python3.4/dist-packages/cryptography/hazmat/backends/__init__.py", line 28, in _available_backends  
    "cryptography.backends"
  File "/usr/local/lib/python3.4/dist-packages/cryptography/hazmat/backends/__init__.py", line 27, in <listcomp>
    for ep in pkg_resources.iter_entry_points(
  File "/usr/local/lib/python3.4/dist-packages/pkg_resources/__init__.py", line 2293, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/usr/local/lib/python3.4/dist-packages/cryptography/hazmat/backends/openssl/__init__.py", line 7, in <module>
    from cryptography.hazmat.backends.openssl.backend import backend
  File "/usr/local/lib/python3.4/dist-packages/cryptography/hazmat/backends/openssl/backend.py", line 52, in <module>
    from cryptography.hazmat.bindings.openssl import binding
  File "/usr/local/lib/python3.4/dist-packages/cryptography/hazmat/bindings/openssl/binding.py", line 173, in <module>
    Binding.init_static_locks()
  File "/usr/local/lib/python3.4/dist-packages/cryptography/hazmat/bindings/openssl/binding.py", line 137, in init_static_locks
    cls._ensure_ffi_initialized()
  File "/usr/local/lib/python3.4/dist-packages/cryptography/hazmat/bindings/openssl/binding.py", line 124, in _ensure_ffi_initialized
    cls.lib = build_conditional_library(lib, CONDITIONAL_NAMES)
  File "/usr/local/lib/python3.4/dist-packages/cryptography/hazmat/bindings/openssl/binding.py", line 89, in build_conditional_library
    if not getattr(lib, condition):
AttributeError: cffi library '_openssl' has no function, constant or global variable named 'Cryptography_HAS_SET_ECDH_AUTO'