Fix gemspec bundle error in Ruby 1.9 by not using require_relative.

require_relative 'lib/dnsruby/version' works in Ruby versions >= 2.0,
but in 1.9 results in the following error:

```
There was a LoadError while loading dnsruby.gemspec:
cannot infer basepath from
/Users/kbennett/work/dnsruby/dnsruby.gemspec:1:in `require_relative'

Does it try to require a relative path? That's been removed in Ruby 1.9.
```

Fixed by using require and building the filespec ourselves.