Fix version comparison with RC strings (e.g. 1.5.0-RC1)

Fixes this stack trace using Foreman 1.5.0-RC1 or the 1.5-stable branch:

<pre>
[root@foreman01 ~]# foreman-rake db:migrate --trace
** Invoke db:migrate (first_time)
** Invoke environment (first_time)
** Execute environment
rake aborted!
Malformed version number string 1.5.0-RC1
/opt/rh/ruby193/root/usr/share/rubygems/rubygems/version.rb:187:in `initialize'
/opt/rh/ruby193/root/usr/share/gems/gems/staypuft-0.0.9/app/models/staypuft/concerns/hostgroup_extensions.rb:57:in `new'
/opt/rh/ruby193/root/usr/share/gems/gems/staypuft-0.0.9/app/models/staypuft/concerns/hostgroup_extensions.rb:57:in `<module:HostgroupExtensions>'
/opt/rh/ruby193/root/usr/share/gems/gems/staypuft-0.0.9/app/models/staypuft/concerns/hostgroup_extensions.rb:1:in `<top (required)>'
</pre>

Remember to re-tag any new release to fix this issue into the foreman-plugins-1.5-* tags so it's accessible for 1.5 users.