Use safe_load function to load YAML

In our use case, we are *not* reading arbitrary input that could be
malicious. Still, because we know that what we're reading is made up of
only dictionaries and lists and not arbitrary Python objects, we might
as well use the more restrictive `safe_load` function rather than the
`load` function.

dotbot/config.py

@@ -8,7 +8,7 @@ class ConfigReader(object):
     def _read(self, config_file_path):
         try:
             with open(config_file_path) as fin:
-                data = yaml.load(fin)
+                data = yaml.safe_load(fin)
             return data
         except Exception as e:
             msg = string.indent_lines(str(e))