Add license in .gemspec

Rubygems warns empty license

```
spec/quality_spec.rb
```

WARNING:  licenses is empty

Warn is licenses is empty. Fixes #202 · ed1098d · rubygems/rubygems
https://github.com/rubygems/rubygems/commit/ed1098d5f14f2d2e97b2ac85d839915bf4c88161