fix berkshelf link for update api

This link corrects the api link for berkshelf and prevents this warning

$ rspec
DEPRECATED: Your Berksfile contains a site location pointing to the Opscode Community Site (site :opscode). Site locations have been replaced by the source location. Change this to: 'source "http://api.berkshelf.com"' to remove this warning. For more information visit https://github.com/berkshelf/berkshelf/wiki/deprecated-locations
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