We're experiencing failed CI builds that are related to apt errors with broken dependencies or missing packages. This is probably related to the fact that Xenial is now the default build environment and our Travis configuration still uses some hacky workarounds for Trusty.

Probably also a good opportunity for a couple updates. On the top of my head:

• explicitly pin OS version to Xenial
• remove g++-6 support in CI build (we already have 7 and 8)