travis: Try downgrading virtualbox

Not sure if it's going to help, but why not?

Author: sfalexrog

.travis.yml

@@ -4,7 +4,7 @@ before_install:
 - wget https://www.virtualbox.org/download/oracle_vbox_2016.asc -O- | sudo apt-key
   add - && sudo sh -c "echo deb https://download.virtualbox.org/virtualbox/debian
   bionic contrib >> /etc/apt/sources.list" && sudo apt-get update && sudo apt-get
-  install build-essential gcc make linux-headers-$(uname -r) virtualbox-6.1 && sudo
+  install build-essential gcc make linux-headers-$(uname -r) virtualbox-6.0 && sudo
   usermod -aG vboxusers $(whoami)
 - mkdir output-virtualbox-iso && pushd output-virtualbox-iso && wget --progress=dot:giga https://github.com/sfalexrog/clever_vm/releases/download/v0_base_vm/clever-devel.ova
   && popd