Fix old command for heroku toolbelt

`heroku config:add` works but it is previous version's.
* https://devcenter.heroku.com/articles/config-vars#setting-up-config-vars-for-a-deployed-application

So I rewrite `config:add` with `config:set`.
Merge pull request #640 from tricknotes/fix-old-command

Fix old command for heroku toolbelt