Include list of pre-requisites in README.md

When I executed the instructions as-is on OS X 10.8.4, I received:

$ rake install_plugins
(in /Users/webchick/proviso)
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rake.rb:2383: command not found: vagrant plugin list

Presumably, this is because I didn't have vagrant installed. Improving the text to help the next person.