build: Updates supported distributions.

[tgockel]

• Arch
  • Works again...the Docker Hub image was renamed to archlinux/base
• Debian 9.9
  • Now done directly as debian:9.9 instead of through debian:9
  • Uses the g++ package directly, removing the need for update-alternatives
• Fedora 27
  • Removes use of update-alternatives, as gcc-c++ does this already
• Fedora 30
  • Added support
• openSUSE 42.3
  • Removes support (use openSUSE 15.1)
• openSUSE 15.1
  • Added support
  • Note that openSUSE does not have packaging support yet (see #126)
• Ubuntu 16.04
  • Use GCC 5 (system default) instead of GCC 7 from the PPAs
• Ubuntu 17.10
  • Removes support (EOL)
• Ubuntu 18.04
  • Uses the g++ package directly, removing the need for update-alternatives
• Ubuntu 18.10
  • Added support
• Ubuntu 19.04
  • Added support

Fixes issue #125.