Use Component#verify only for classes

This fixes `TypeError: RPM is not a module` when some of the dependencies are not met:
```
> require 'polisher'
TypeError: RPM is not a module
	from /home/strzibny/.gem/ruby/gems/polisher-0.10.2/lib/polisher/rpm/has_requirements.rb:10:in `<module:Polisher>'
	from /home/strzibny/.gem/ruby/gems/polisher-0.10.2/lib/polisher/rpm/has_requirements.rb:9:in `<top (required)>'
	from /usr/local/share/ruby/site_ruby/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /usr/local/share/ruby/site_ruby/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /home/strzibny/.gem/ruby/gems/polisher-0.10.2/lib/polisher/rpm/spec.rb:9:in `<top (required)>'
	from /usr/local/share/ruby/site_ruby/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /usr/local/share/ruby/site_ruby/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /home/strzibny/.gem/ruby/gems/polisher-0.10.2/lib/polisher.rb:9:in `<top (required)>'
	from /usr/local/share/ruby/site_ruby/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /usr/local/share/ruby/site_ruby/rubygems/core_ext/kernel_require.rb:73:in `require'
	from (irb):3
	from /usr/bin/irb:12:in `<main>'
```
After the fix:
```
> require 'polisher'
Failed to require versionomy.  Added runtime exception in Polisher::RPM::Requirement
Failed to require versionomy.  Added runtime exception in Polisher::RPM::Spec
Failed to require pkgwat.  Added runtime exception in Polisher::Bodhi
Failed to require pkgwat.  Added runtime exception in Polisher::RHN
Failed to require pkgwat.  Added runtime exception in Polisher::Fedora
=> true
```