Bump knife-opc to 0.2.0

Ping @opscode/server-team 

Users now have a way to add themselves to admins via `chef-server-ctl` https://gist.github.com/tylercloke/6502109f47eeea3575da

[Build](http://wilson.ci.opscode.us/job/chef-server-12-trigger-ad_hoc/121/downstreambuildview/)