Add #to_s method to VERSION modules

IMO this is pretty useless behaviour:

```ruby
>> Rails::VERSION
=> Rails::VERSION
>> ActiveSupport::VERSION
=> ActiveSupport::VERSION
```

This patch adds a `#to_s` method so that accessing `*::VERSION` gives useful output in REPLs, etc:

```ruby
>> Rails::VERSION
=> 4.0.0.beta
>> ActiveSupport::VERSION
=> 4.0.0.beta
```