Move version to separate file, remove require_relative from gemspec

I got the following error:

```
$ bundle update sdoc
Fetching git://github.com/voloko/sdoc.git
There was a LoadError while loading sdoc.gemspec: 
cannot infer basepath from
  /Users/manuel/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/bundler/gems/sdoc-1a0e80c2d629/sdoc.gemspec:2:in `require_relative'

Does it try to require a relative path? That's been removed in Ruby 1.9.
```

So I removed the `require_relative` from the gemspec and in the same process moved the version constant into a separate file so only that that needs to be loaded.