ncbi: entrez_fetch causing build failure

[stephenturner]

The following lines seems to be causing a build failure.

recs <- entrez_fetch(db = "pubmed", id = res$ids[1:25], rettype = "xml", parsed = TRUE)

recs <- entrez_fetch(db = "pubmed", web_history = res$web_history, rettype = "xml", parsed = TRUE)

They both work fine when run interactively but throw the following error when trying to knit the Rmd.

Error in entrez_check(response) : 
  HTTP failure: 502, bad gateway. This error code is often returned when trying to download many records in a single request.  Try using web history as described in the rentrez tutorial
Calls: <Anonymous> ... entrez_fetch -> do.call -> <Anonymous> -> entrez_check
Execution halted

Exited with status 1.

I turned these off with eval=FALSE for now.

cc @vpnagraj
see also #43