Fix deprecations

[mikicz]

/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/flask_session/sessions.py:313
  /home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/flask_session/sessions.py:313: DeprecationWarning: 'werkzeug.contrib.cache' is deprecated as of version 0.15 and will be removed in version 1.0. It has moved to https://github.com/pallets/cachelib.
    from werkzeug.contrib.cache import FileSystemCache

/home/travis/build/pyvec/naucse-hooks/naucse_hooks.py:76: YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.
    fork = yaml.load(child.read_text())

[mikicz]

Second warning was fixed with db02158