Update ruby version to 2.0.0-p481.

Looks like we're not using 2.0.0-p384 on Travis (https://travis-ci.org/rubygems/rubygems.org/jobs/31390764#L23) and it looks like p384 isn't even installable:

```
curl: (22) The requested URL returned error: 404
The requested url does not exist(22): 'http://cache.ruby-lang.org/pub/ruby/2.0/ruby-2.0.0-p384.tar.bz2'
Checking fallback: http://ftp.ruby-lang.org/pub/ruby/2.0/ruby-2.0.0-p384.tar.bz2
Checking fallback: http://www.mirrorservice.org/sites/ftp.ruby-lang.org/pub/ruby/2.0/ruby-2.0.0-p384.tar.bz2
```

http://cache.ruby-lang.org/pub/ruby/2.0/