Use yaml.safe_load instead of yaml.load

See my comment in https://github.com/orchardup/fig/pull/76#issuecomment-48610330

Fig maybe should use `yaml.safe_load` not `yaml.load`.  It's kinda powerful, but in a way that makes me think it's probably insecure.

http://pyyaml.org/wiki/PyYAMLDocumentation#LoadingYAML
Signed-off-by: Richard Morrison <richard@rmorrison.net>