Fix setup via vagrant

The Vagrantfile used an outdated VM base image (Ubuntu Precise 12.04)
that is no longer available/supported. This commit will make vagrant
use Ubuntu 18.04 (20.04 is not yet supported by the ocaml PPA).
The VM is setup via running scripts/travis-ci.sh, but I found that
additional packages are needed in order to run `vagrant up`
successfully with the new base image. Also one package has been
renamed in newer Ubuntu versions (and now also comes preinstalled).

Vagrantfile

@@ -2,8 +2,7 @@
 # vi: set ft=ruby :
 
 Vagrant.configure("2") do |config|
-  config.vm.box = "precise64"
-  config.vm.box_url = "http://files.vagrantup.com/precise64.box"
+  config.vm.box = "ubuntu/bionic64"
 
   config.vm.provision :shell, :path => "scripts/travis-ci.sh"
 

scripts/travis-ci.sh

@@ -15,7 +15,9 @@ else
         }
         # update repositories to prevent errors caused by missing packages
         sudo apt-get update -qq
-        apt python-software-properties # needed for ppa
+        apt gcc
+        apt libgmp-ocaml-dev
+        apt software-properties-common # needed for ppa
         apt make m4  # needed for compiling ocamlfind
         apt patch    # needed for compiling xml-light
         apt autoconf # needed for compiling cil