fix 'cannot load such file -- args_parser' error

I found ```irkit``` command raises ```LoadError```.
So this pull-req fixes the problem.

    $ irkit --help
    /Users/mallowlabs/.rbenv/versions/2.0.0-p353/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:53:in `require': cannot load such file -- args_parser (LoadError)
        from /Users/mallowlabs/.rbenv/versions/2.0.0-p353/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:53:in `require'
        from /Users/mallowlabs/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/irkit-0.0.5/bin/irkit:6:in `<top (required)>'
        from /Users/mallowlabs/.rbenv/versions/2.0.0-p353/bin/irkit:23:in `load'
        from /Users/mallowlabs/.rbenv/versions/2.0.0-p353/bin/irkit:23:in `<main>'