fix failing test
fix failing test

- fixes #251
- see discussion at 
https://github.com/cmu-delphi/delphi-epidata/issues/251
- in short, the test inserted bad data into the database. "bad" in the 
sense that the latest for for "sig2" was not marked as such by the 
`is_latest_issue` field
- the fix is to set that field on the "sig2" row

cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata