GR: use correct attribute for axis color

[greimel]

using Plots
using StatPlots, DataFrames

test = DataFrame(
    x = rand(20),
    y = rand(20),
    grp = rand(["g 1"; "g 2"], 20)
 )

gr()
@df test scatter(:x, :y, group=:grp, xlab="x", ylab="y",
                  foreground_color_axis=:blue,
                  foreground_color_border=:green)

Before:

After:

Now it matches behavior for pyplot(), I can't reproduce right now, because my PyCall.jl doesn't build.

This will be helpful for JuliaPlots/PlotThemes.jl/pull/30

As an aside, the attribute names could be changed so something more intuitive, IMO.

[daschw]

Thanks @greimel !