Define Rails version dep in one place

Just upgrading rails to the latest 3.x version.

**NB: Don't land this just yet. Changes forthcoming. Please hold. Thank you for your cooperation in this matter.**

## Verification steps

- [ ] All specs pass. That's it.
Once `Metasploit::Framework::RailsVersionConstraint::RAILS_VERSION` is an `Array`, this will need to be splatted:

```ruby
spec.add_runtime_dependency 'rails', *Metasploit::Framework::RailsVersionConstraint::RAILS_VERSION
```