Require the latest version of Vagrant

In README it is not stated which version of vagrant to use. I installed the one from rubygems.org which appears to be a very old one. The exact problem I faced was [this](https://github.com/wafflesfm/batter/issues/1) when I did `vagrant up`. I installed the latest version and the error went away. 

I also included a minor identation fix of the requirements list :)