Add evalcast dependencies

[tildechris]

Closes #6.

Also switch to rocker/tidyverse base image because of problems installing evalcast from miniconda.

[tildechris]

@JedGrabman, after installing evalcast, I'm running the create_reports.R from the Reports directory, but there's a undefined symbol:

could not find function "get_covidhub_forecaster_names"

Is there a specific version/branch of evalcast that we should be using?

[tildechris]

@kateharwood, I think this is working now. Can you try running make and see if it updates the rds. On my machine, it takes about 10 minutes to run.