Fix issue, HerokuAddonManager#retrieve now returns an AddonPlan

That fixes an issue that caused before and current to always be
different in AbstractControllerPlugin#run

That'll prevent reaching heroku' API rate limit
remove stupid log
Add specs for wrong value type returned by HerokuAddonManager#retrieve
Merge pull request #6 from harrystech/fix_addon_retrieve_type

Make HerokuAddonManager#retrieve returns an AddonPlan