Handle case when other envs are missing a cookbook.

* Refactored to use the same method to extract version strings.
* Using 'none' as a placeholder for missing version.

lib/chef/knife/env-diff.rb

@@ -45,12 +45,10 @@ def run
         from_cookbooks.each do |from_cookbook, from_data|
           diff_versions = {}
 
-          unless from_data['versions'].empty?
-            from_version = from_data["versions"].first['version'] 
-          end
+          from_version = get_cookbook_version(from_data)
 
           to_env.each do |to_env, to_cookbooks|
-            to_version = to_cookbooks[from_cookbook]["versions"].first['version']
+            to_version = get_cookbook_version(to_cookbooks[from_cookbook])
 
             if from_version != to_version || from_version.nil?
               diff_versions[to_env] = to_version
@@ -65,15 +63,20 @@ def run
             end
             ui.msg "\n"
           end
-
         end
       end
-
     end
 
-   def get_env_cookbooks(env)
+    def get_env_cookbooks(env)
       rest.get_rest("environments/#{env}/cookbooks")
-   end
+    end
 
+    def get_cookbook_version(data)
+      if data['versions'].empty?
+        'none'
+      else
+        data['versions'].first['version']
+      end
+    end
   end
 end