add require version

fix below error

% knife softlayer
/home/d-higuchi/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/knife-softlayer-0.0.3/lib/chef/knife/softlayer_base.rb:14:in `<module:SoftlayerBase>': uninitialized constant Knife (NameError)