Starting July 17th, 2017 all new Intuit Developer accounts will need to use OAuth2 for API access. Here is how to use the QboApi gem and OAuth2.
Choosing an OAuth2 gem or not
I really won't bother trying to re-invent the wheel by rolling your own OAuth2 code. That said, the OAuth2 2-legged process is simpler than the 3-legged OAuth 1a process and if you do want to roll your own OAuth2 code take a look at Intuit's Python example for a good starting point.
As for me, I'll choose to leverage the Rack-OAuth2 gem as I like its ability to directly set endpoints.
Spinning up an OAuth2 example
Clone the qbo_api gem, switch into the new directory, and bundle
$ git clone git://github.com/minimul/qbo_api && cd qbo_api
Create a .env file with the client_id and client_secret provided within the App settings page. See Fig. 2.