Base.banner_color -> Base.banner()

After https://github.com/JuliaLang/julia/commit/2af216990aa7e94dc47b2fb4f2e697c7741584ae, the banner should be printed from the `Base.banner()` function instead of the `Base.banner_color` and `Base.banner_plain` strings. Unfortuenatly `have_color` is not set correctly in the new REPL, but that is the next step.