yaml.load() is unsafe

[davidwagner]

Running jupyter-book build triggers the following warnings:

.../jupyter_book/build.py:130: YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.
  site_yaml = yaml.load(ff.read())
.../jupyter_book/build.py:139: YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.
  toc = yaml.load(ff.read())

Perhaps those yaml.load() calls could be using yaml.safe_load()?

[emdupre]

Thanks for reporting, @davidwagner ! This should be fixed when #235 is merged.