WPScan is a useful tool for finding vulnerabilities in your WordPress blog. Here are the steps I followed to install wpscan on centos 6.3.

1. Install Ruby 1.9.x and RubyGems 1.8.24 from the source: http://www.rosehosting.com/blog/how-to-install-ruby-1-9-3-and-rubygems-1-8-24-on-centos-6-2/

2. I also needed to install 'readline'

 sudo gem install rb-readline

3. Steps to install and run WPScan: https://github.com/wpscanteam/wpscan/blob/master/README.md