Update base box to use 18.04

[jonathanbossenger]

I ran the build scripts last night (using the lamp install) but with the 18.04 bento box, and everything just worked. I'm going to try the nginx version tonight, and if it all works, I'll submit a PR to update the Vagrant file.

All of which you can obviously ignore if you like, I just like to use the latest Ubuntu versions ;-)