updated github background color to their "dark grey" color per https://styleguide.github.com/primer/utilities/colors/
updated github background color to their "dark grey" color per https://styleguide.github.com/primer/utilities/colors/

Thank you @eclectic-coding for bringing up this issue in https://github.com/jaketrent/react-social-icons/pull/54

jaketrent/react-social-icons,jaketrent/react-social-icons