Periodically update pubmed source data automatically

[peetucket]

Check to see if the Pubmed HEAD request contains cache invalidation data that we can track to detect updates (or perhaps an update date field). If there is, we can use the updated_at field in the PubmedSourceRecord data, we could run a rake task over all the Pubmed data, say monthly to update PubMed data and then update pubhashes

We should also update the pub hash and identifiers in the PublicationIdentifiers table