Allows ruby_version to be an array

There are some use cases where you might want to specify several ruby versions for installing a ruby gem. Currently you're required to specify the gem multiple times if you want to exclude a specific ruby version(s). I thought it might be useful to also allow an array to be set for the ruby versions.

Some more thoughts on this: It might also be useful/be better to just add an `except` or `excludes` method that excludes some ruby versions. This might be a better approach to this problem, but I've got no idea how to implement this.

**Please note**: I wasn't able to test this, because I'm not entirely sure how to test this. I've got (close to) zero experience with these puppet/boxen packages. So if you think this is a nice feature, do you want to help me out with writing the specs for this?

**Additional footnote:** I believe the puppet version should be updated too, because running rspec gives the error message described in this issue: [Puppet raises an exception under Ruby 2.1](https://tickets.puppetlabs.com/browse/PUP-1266).