Add check for published_on in exim.py

If you don't get into the if `temp:` block, `published_on` is not set, which leads to an error.

The offending, unprocessable link (from http://www.exim.gov/oig/index.cfm):
![photo](https://api.monosnap.com/image/download?id=cFPH7C5XpXMXrV5YntJaMNEXfNCsEr)