Add Ruby modeline to Vagrantfile

Editor which support Emacs modelines (e.g. SublimeText with a plugin
and Emacs itself) automatically enable Ruby syntax highlighting when
visiting the Vagrant configuration file.
Use box-cutter/debian75 for both VirtualBox and VMware

Use a single Vagrant base box since the one made by Box-Cutter supports both
VirtualBox and VMware (Workstation and Fusion) hypervisors.
Use proper configuration directives to tune VM memory
Vagrantfile: Add entries to test/develop against Ubuntu 12.04 and 14.04