Switch Ruby packaging from Rakefile to gemspec

Currently the packaging is done via a Rakefile. However, the various parts of Rake have turned into a moving target with incompatible changes. So for packaging the gems we should move packaging to use a gemspec instead.

In the process, we can also dispose of the Qpid::Messaging::Version class which is not used by anything except the Rakefile. We can instead set the version directly in the gemspec file.