NO-JIRA: Removed Rakefile in favor of a gemspec file.

Since the Rakefile brought in dependencies that were ultimately
unnecessary, packaging was simplified to just using a gemspec file.

Updated the release.sh script to use the gemspec file.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1429987 13f79535-47bb-0310-9956-ffa450edef68