Remove require from gemspec

By requiring `lib/venice.rb` in the gemspec, it means all the gems in the bundle have to be installed before the bundle itself can be installed.

```
There was a LoadError while evaluating venice.gemspec: 
cannot load such file -- excon from
  /Users/Larry/Code/venice/venice.gemspec:3:in `<main>'

Does it try to require a relative path? That's been removed in Ruby 1.9.
```

There are numerous ways to pull in the version from a constant without requiring the entire library. I stole this from [faraday](https://github.com/lostisland/faraday/blob/75ab43c2ea6d27604b33c53e0a02cad5d600aa39/faraday.gemspec#L1-L4).