I'm not a ruby user. However, if using gem is the most common way to install a ruby library and nobody needs a debian package, then we should remove ruby completely from the debian package build.
Strong -1 from a Debian developer here. While it's common for Ruby developers to get libraries as gems during development and testing, there are still a lot of good reasons for sticking to OS packages for deployment. Ideally, both a thrift gem and a ruby-thrift deb should be available.
Commenting out erlang from debian build until jsx can be fixed.
I recommend you remove the corresponding stanza from debian/control as well, or you'll get confusing empty libthrift-erlang packages lying around.