s:Process.system_bg() was renamed (thanks @basyura)

37717cb causes an error:

    E716: Key not present in Dictionary: system_bg

because s:Process.system_bg() was renamed to s:Process.spawn().

thanks @basyura for bug report.
https://github.com/tyru/open-browser.vim/commit/37717cb49b0cfaa05fa9c16a5c3759ca5e3ebdf2#commitcomment-3884304