[ERROR] Install missing ruby gem when running wpscan

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http://www.backtrack-linux.org – Dears all When I tried to enumerate a web using wpscan I got an error as the following: root@abrao:/pentest/web/wpscan# ruby wpscan.rb -h [ERROR] Install missing ruby gem. Please see README file or http://code.google.com/p/wpscan/ #<LoadError: no such file to load -- tools/generate_plugin_list> How to solve this problem...?? Thanks (HowTos)