Fix conversion of config timestamp to datetime

It was using `fromtimestamp` which 'returns the local date and time'.
The `utcfromtimestamp` returns a UTC datetime.
This probably explains the reason for the inconsistency seen in #110.

django_publicdb/histograms/jobs.py

@@ -521,7 +521,7 @@ def update_config(summary):
             elif var in ['mas_version', 'slv_version']:
                 vars(new_config)[var] = blobs[config[var]]
             elif var == 'timestamp':
-                ts = datetime.datetime.fromtimestamp(config[var])
+                ts = datetime.datetime.utcfromtimestamp(config[var])
                 vars(new_config)[var] = ts
             else:
                 vars(new_config)[var] = config[var]