Add config/ to gemspec

The config directory is missing from the gemspec, and as such, the config
directory is missing when you install rubocop-rspec. This results in errors:

```
Running RuboCop...
No such file or directory @ rb_sysopen - /Users/pstengel/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rubocop-rspec-1.0/config/default.yml
```