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 'activesupport', '>= 3.0.0',  *Metasploit::Framework::RailsVersionConstraint::RAILS_VERSION
```

It may be better to roll the '>= 3.0.0' into `Metasploit::Framework::RailsVersionConstraint::RAILS_VERSION` and just have

```ruby
module Metasploit
  module Framework
    module RailsVersionConstraint
      # The Metasploit ecosystem is not ready for Rails 4 as it uses features of
      # Rails 3.X that are removed in Rails 4.
      RAILS_VERSION = ['>= 3.0.0', '< 4.0.0']
```

```ruby
spec.add_runtime_dependency 'activesupport', *Metasploit::Framework::RailsVersionConstraint::RAILS_VERSION
```