Update ohai_plugin recipe to use rbenv_bin_path

to bring into line with 31c1ad9c6ed2dbbf80bebf02d11b903932c5425a
Restore actual rbenv binary to path in ohai_plugin
Merge pull request #61 from donaldguy/ohai_fix

ohai_plugin broken, 31c1ad9c6 removed rbenv_binary_path but recipe still uses