Correctly corrected sensu-plugin gem requires

Sensu was unable to find the sensu-plugin gem.  It needed to get installed
into the system path, not the user gem path.  This explains why the manual
gem install on demo-1 corrected the issue.  Also, the manual install installed
gem deps (e.g. json).

    output /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- sensu-plugin/check/cli (LoadError) from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require' from /etc/sensu/plugins/check-log.rb:18:in `<main>'