Fixed a problem in the motd::knife-status recipe

When it gets the "interval" value from `node['chef_client']['interval']`, it was failing to convert the seconds to minutes since the value returned was actually a string. This fix just ensures that the value in "interval" is actually an integer, so that we can do math operations on it.

See: <https://github.com/opscode-cookbooks/chef-client/blob/master/attributes/default.rb#L36>