Use yaml.safe_load() instead of load()

[lbeaufort]

Resolves #278

Use yaml.safe_load() instead of load(). load() has known security issues: yaml/pyyaml#189. Tests pass with invoke test - please let me know if you'd like me to make any more changes. I wasn't sure if I should add myself to AUTHORS.rst.

Thanks!
Laura

cc: @sloria and @lafrech

[sloria]

@lbeaufort Thanks! Please do add yourself to AUTHORS.rst. Gotta give credit where it's due =).

[lbeaufort]

@sloria thanks! 😄I updated my commit.

[sloria]

Thanks!