Updated tests to run on all supported platforms

I updated the tests to run on all four supported platforms.
To run the tests you need at least vagrant 1.5 and optionally pytest installed (disponible via pip).
If you use pytest, simply change into the test directory and run `py.test`, it will run the four tests, downloading boxes if necessary.
I didn't update the readme in case my approach doesn't suites you.

I also updated the fabfile to install `software-properties-common` since I ran into the problem while testing