Adding flavor names to README.md
Fixing version; specified tag for vagrant in Gemfile as the current mainline does not work with vagrant-rackspace.
Updated example to use performance flavor.
Merge pull request #70 from rackspace/flavor_doc

Updated README to reflect performance flavors This PR updates the README to include a list of current flavors on the Rackspace Cloud as well as the README example to use performance flavors.