Fix referencing consts from RPM::Spec

Trying to update the spec file after all my other submitted fixes are used I still get:
```
$  ~/projects/polisher/bin/ruby_rpm_spec_updater.rb rubygem-sup.spec 
/home/strzibny/.gem/ruby/gems/polisher-0.10.2/lib/polisher/rpm/updates_spec.rb:98:in `update_metadata_contents': uninitialized constant Polisher::RPM::UpdatesSpec::SPEC_VERSION_MATCHER (NameError)
```
This PR fixes the issue.