Travis-ci: bump linux base system 'dist' up to 'bionic'.
Travis-ci: bump linux base system 'dist' up to 'bionic'.

darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed