Ubuntu Bionic improvements for ARM+LXD builds

We have updated the Bionic 18.04 OS image to run within LXD containers on ARM infrastructure.

Changes:

  • fixed link to llvm-symbolizer-6.0
  • running docker commands without sudo
  • added g++ and gcc packages (the previous version had only g++-7 and gcc-7)

To enable testing on multiple CPU architectures, just add the arch: key to your .travis.yml:

arch:
  - amd64
  - arm64
os: linux  # different CPU architectures are only supported on Linux

You can learn more in the multi CPU architecture support announcement and in multi CPU architectures docs.