Auto merge of #1678 - chris-morgan:master, r=alexcrichton

This regression, introduced in d3f590f59ff72f1d3b457ed3716483ce472053c6 in #1674, caused `make` to fail on machines where `python` is Python 3 rather than Python 2, because `dict.keys()` returns a set-like object that does not support indexing rather than a list.

`date = next(iter(snaps.keys()))` would be an adequate fix too, but I’m not particularly happy with either of them, for neither would stand up to the removal of the `break` statement: one is inadequate because dictionaries are unordered collections, the other is inadequate because `current` would refer to the *last* rather than the *first* snapshot in the file.