Updating Craft 3 from ver. 3.0.28 10/23/2018
You need to manually update to the latest version of Craft 3.0.x, then update to Craft 3.1.x:


In your composer.json, change your Craft package to "craftcms/cms": "3.0.40",
Run composer update
Then in your composer.json, change your Craft package to "craftcms/cms": "^3.0.40",
Run composer update again