foreman-proxy needs to depend on ruby-rgen on trusty

otherwise it even won't start:
```
root@tmptst:~# service foreman-proxy start 
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- rgen/metamodel_builder (LoadError)
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/vendor_ruby/puppet/pops/model/model.rb:23:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/vendor_ruby/puppet/pops.rb:20:in `<module:Pops>'
	from /usr/lib/ruby/vendor_ruby/puppet/pops.rb:3:in `<module:Puppet>'
	from /usr/lib/ruby/vendor_ruby/puppet/pops.rb:1:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/share/foreman-proxy/lib/proxy/puppet/class_scanner_eparser.rb:4:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/share/foreman-proxy/lib/proxy/puppet/puppet_class.rb:3:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/share/foreman-proxy/lib/proxy/puppet.rb:6:in `<module:Puppet>'
	from /usr/share/foreman-proxy/lib/proxy/puppet.rb:3:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/share/foreman-proxy/lib/proxy.rb:17:in `<module:Proxy>'
	from /usr/share/foreman-proxy/lib/proxy.rb:1:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/share/foreman-proxy/lib/smart_proxy.rb:5:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/share/foreman-proxy/bin/smart-proxy:5:in `<main>'
```