cpc_prcp: bug in some years (dates < 2008-12-31)

[sckott]

related to #242

thought I had fixed this, but user reported problem back in November 2017 - finally gettting to this

Results are not turning out correctly, not sure why yet.

e.g.

foo <- function(x) {
  tmp <- cpc_prcp(date = x, us=FALSE)
  p <- ggplot(tmp, aes(x=lon, y=lat, fill=precip)) + theme_bw()
  p + 
    geom_tile() +
    ggtitle(sprintf("cpc_prcp(date = '%s', us=FALSE)", x))
}

foo("2008-12-31")

foo("2009-01-01")