Remember that Ruby is a very reflective and dynamic language. Once an encrypted RBE is loaded, the code is loaded into the interpreter and there are many ways to poke and inspect the various objects - like strings used, that might contain sensitive data. Any language with a garbage collector will make data stay alive for longer than the execution of the code.
Also, there are various tools meant to debug HTTP traffic, that also allow the administrator of a machine to also inspect HTTPS traffic. This means the secret key passed from the machine could be extracted from the owner of the machine.
I had some PHP experience from before. So I set up a new server for my commercial extensions using Laravel, a PHP framework. Then I used Laravel Forge to help me deploy the website easily by just pushing to a Git repository for the website.
That being said, if you only want to have a license server, you can go for a simpler solution. Find a host that offer any language you prefer, Ruby, PHP, Python… what ever suit your needs. If Ruby is your strong suit, then you might be better off picking a host that allows you to use Ruby. And then you should be able to set up a rather small license server. Just an API endpoint which you call from your extension, then your server makes the SendOwl request on you behalf. This could keep your secret key entirely secured on your own server and not stored in any form on your users’s machine.
I’m not that familiar with Ruby web applications, but Rack might be one solution to quickly set up a webserver: https://rack.github.io/
But this is just a suggestion. I merely wanted to point out potential gotchas in case you were not aware of them. How much effort you want to put into it is up to you.
I’m in favor of not spending too much time on license and anti-piracy measure. Just enough to keep honest people honest. You can ramp up the security if you see it actually being a problem. Otherwise it’s very easy to get lost in bolting on too much security measure without actually getting to the point of delivering a product.