Use test-kitchen for multiplatform testing

[gravitystorm]

We should use test-kitchen to test that the recipes run on different platforms and versions of chef. We can't just make changes (e.g. #18 ) since other installations of cyclescape might not be on the same platform as the cyclescape.org instance, and might not be able to upgrade when we do. The current approach of firing up a virtual machine, running the cookbooks by hand and 'seeing if they work' is somewhat time-consuming and error prone!

We should support Ubuntu 12.04 and Ubuntu 14.04.

It's not clear how feasible this is before sorting out #19 but I'll try it anyway.