Version checking to prevent silent compatibility issues

My esteemed colleagues,

I propose a pull request to resolve #31. This pull request:

 * Gets rid of that warning about Rake version in `arkenstone.gemspec`
 * Lock users out of `Rake` if they are on old versions of Ruby that won't work anyway.
 * set the `required_ruby_version` in the `gemspec` to anything >= 2.0