Move $VERSION to Mojo::Base, so it is available for each Mojo's package

Could you move $VERSION to Mojo::Base package, please?

This variable is accessible only by Mojolicious package which is unnecessary and problematic by CLI scripts. I noticed that Mojo::Base could be the best option as far as it is required by any Mojo's package.

It would be great if I could write:

```perl
use Mojo::Base 5.67 -strict;
use Mojo::UserAgent;
my $tx = Mojo::UserAgent->new->get('http://mojolicio.us');
...
```