Updates locate_config_file scope to reflect its change to a class method in commit 2a214bc

The locate_config_file was changed to  a class method on  2013-10-09:
https://github.com/opscode/chef/commit/2a214bcff7174b61129a7a8a642b8df5b5f4afee#diff-1220e994288a7f7d9ae83196ce7fad9eL317

This fixes the scope and uses an abbreviated assignment for  config[:config_file]

Note: I did not bump the gem tag.

Thanks!