Added boost (BH) to environment.yml

[jeanimal]

Without this addition, I get a "Boost not found" error from Stan when running base.r.

$ Rscript base.r base
…<snip>...
[1] "First non-zero cases is on day 60, and 30 days before 10 deaths is day 46"
[1] "Netherlands has 71 days of data"
Error in stan_model(paste0("stan-models/", StanModel, ".stan")) : 
  Boost not found; call install.packages('BH')
Execution halted

[jeanimal]

I see the error in docker. I do not know much about docker, but I will try to figure it out.

[jeanimal]

The docker failure is unrelated to my change. I have had docker problems with every version of the code I checked out, but unfortunately I don't know enough about docker to fix it.