Modified timestamp includes a decimal point

We noticed that the modified timestamps in the Open Sessions feed include a decimal point. See [this page](https://opensessions.io/API/rpde/session-series?afterTimestamp=1618994838&afterId=6841) for the following example:

```
{
   "id": 3726,
   "state": "deleted",
   "kind": "SessionSeries",
   "modified": 1619176189.0
},
```