Update Epidata client

[brookslogan]

The version of the delphi-epidata client currently in the epiforecast package appears to be broken, with queries such as Epidata$fluview("nat",201050) returning

$result
[1] -1

$message
[1] "database error"

A quick fix for users is to run

environment(Epidata$fluview)$BASE_URL <- "https://delphi.cmu.edu/epidata/api.php"

or to reassign the Epidata object in the package environment to be one from a current version of delphi-epidata.

A proper fix is to update the version of the client used in this package. A cleaner alternative may be to first make the R client in cmu-delphi/delphi-epidata a proper R package, so that it can be expressed as a dependency using Imports: and Remotes:.