update base image to Ubuntu 18.04 bionic

[NeroBurner]

we could update the base image to bionic. We then could use the clang-8 or clang-9 package from the updates repository

apt-cache policy clang-8
clang-8:
  Installed: (none)
  Candidate: 1:8-3~ubuntu18.04.2
  Version table:
     1:8-3~ubuntu18.04.2 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/universe amd64 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 Packages

apt-cache policy clang-9
clang-9:
  Installed: (none)
  Candidate: 1:9-2~ubuntu18.04.2
  Version table:
     1:9-2~ubuntu18.04.2 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/universe amd64 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 Packages

[paleozogt]

@NeroBurner fyi i've updated to clang 9 in master. looking into using bionic next.