cleaning up gemspec

Cleans up gemspec by adding version dependencies, license, homepage, and summary.

Now it's clean when building! Fixes https://github.com/bellycard/napa/issues/130.

```
> gem build napa.gemspec
  Successfully built RubyGem
  Name: napa
  Version: 0.4.0
  File: napa-0.4.0.gem
```

@bellycard/platform