Adds Github Actions workflow for tests

[davidwessman]

• Allows us to test the gem on multiple versions of Ruby

[boazsegev]

Thanks @davidwessman ,

This is a great start... though, I have to admit that there's a lack of tests I the gem (I have testing scripts on my computer, but they require manual testing of the resulting PDFs).

I hope we add more tests later on :)

Thanks!