Joshua Priddle did a great job back in DotBlock accomplishing Ruby binding for WHMCS API. Unfortunately, last signs of activity were 3 years ago. It’s a pity, as gem is great, and works like charm, but it became massively outdated, and piled with pull requests no one merged in the master branch for ages.
The code was published by MIT license on 2011, thus whmcs-api gem will be its descendant, which I’ll try to keep updated as much as possible.
For now it’s being tested on Ruby 2.x and WHMCS 5.3.x without any problems. The source code and any issues are better be reported on the Github repo.