Change method of enforcing minimum ruby version

[thesecretmaster]

Related to #20. I'm trying to write my own gem, and I want to use guard to help with my development. I also have made my own decision to support older ruby versions. Because guard depends on this gem, I'm unable to both use guard and support older ruby versions.

I understand the rational in #20, that you and the maintainers of guard/listen don't want to have to deal with older ruby versions, but I have made my independant desicion that I do. So what I would request is that ruby_dep will no longer force ruby version changes with a minimum version in the gemspec, but that it will force it by throwing an error at runtime which can be suppressed by an enviroment variable by people like me. It's minimum version can simply be set to be the same as guard/listen.

Until that happens, I'm going to have to drop guard from my gemspec and find some hacky workaround.

[thesecretmaster]

Actually, I've just used an older version of ruby_dep. That works, but I still think that it should be possible to run the latest version without a minimum ruby version issue.