Fix YAML loading

Unfortunately replacing `yaml.load` does not work, because it's used in some other places.

Getting the following traceback without the fix:

    Traceback (most recent call last):
      File "cleopatra/scripts/bootstrap.py", line 9, in <module>
        from clay import config
      File "/Users/ca/.virtualenvs/cleopatra/src/clay-flask/clay/__init__.py", line 2, in <module>
        from clay.server import app
      File "/Users/ca/.virtualenvs/cleopatra/src/clay-flask/clay/server.py", line 8, in <module>
        from clay import config
      File "/Users/ca/.virtualenvs/cleopatra/src/clay-flask/clay/config.py", line 180, in <module>
        CONFIG.load()
      File "/Users/ca/.virtualenvs/cleopatra/src/clay-flask/clay/config.py", line 63, in load
        config = self.load_from_file(path)
      File "/Users/ca/.virtualenvs/cleopatra/src/clay-flask/clay/config.py", line 82, in load_from_file
        config = load(file(filename, 'r'))
      File "/Users/ca/.virtualenvs/cleopatra/lib/python2.7/site-packages/yaml/__init__.py", line 93, in safe_load
        return load(stream, SafeLoader)
    TypeError: safe_load() takes exactly 1 argument (2 given)