(CPR-8) Rake task for signing gems

This commit adds a sign_gem task in gem.rake that unpacks, sets the
signing key and adds the cert to the chain for the spec file. These are
passed in by environment variables RUBY_PRIVATE and RUBY_PUBLIC
respectively. It then rebuilds it thus signing the ruby gems.
This needs to move up a line. We don't want the task available if it build_gem isn't true.