Update from evalcast-killcards to evalcast and reduce memory issues

[JedGrabman]

Requirements:
This PR requires an update from evalcast-killcards to evalcast.

Memory related fixes:
1.) Update from evalcast-killcards to evalcast. Evalcast does not have memoisation, which will reduce memory requirements
2.) Only retrieve forecasts designated as "primary". Reduces memory requirements and aligns evaluation with Reich Lab documentation

Additionally, fixes a bug when processing forecast_dates for a forecaster that has no associated forecasts.

[tildechris]

Error while doing make score_forecast:

Calls to the `evalcast` functions `download_signal()` and
`get_covidhub_predictions()` are memoized in memory by default.
To cache to disk instead use `memoise::cache_filesystem()`. Ex:

 db <- memoise::cache_filesystem("path/to/dir/.rcache")
 download_signal <<- memoise::memoise(download_signal, cache = db)

Error in get_covidhub_forecaster_names(designations = "primary") :
  unused argument (designations = "primary")
Calls: create_prediction_cards
Execution halted

[tildechris]

This is probably because you need to update https://github.com/cmu-delphi/covidcast-docker with the new evalcast package.

[JedGrabman]

@tildechris I've created https://github.com/cmu-delphi/covidcast-docker/pull/1 to address the docker issue, but I don't have permission to set reviewers.