For older Plots versions I found out that it was posssible to set the reversed colormap by c=:viridis_r, but now it generates the error Unknown color: viridis_r:

using Plots
h=0.05
y, x = range(-5, stop=5+h, step=h), range(-5, stop=10+h, step=h)
X = [xx for yy in y, xx in x]
Y = [yy for yy in y, xx in x]
z = (sin.(X)) .^10 + cos.(10 .+ Y .* X) + cos.(X) + 0.2*Y + 0.1*X
heatmap(x, y, z, c=:viridis_r)