Update knowledge_base.gemspec

Fixtures is not being copied to the lib folder, but it's being accessed [here](https://github.com/hyperoslo/knowledge_base/blob/8f67b694b76e76e25089e763bf3fb05c3eb7af97/lib/knowledge_base/seeds.rb#L6).

So after running:

```bash
rake knowledge_base:seed
```

I get this error:

```
rake aborted!
No such file or directory - /Library/Ruby/Gems/2.0.0/gems/knowledge_base-0.1.0/lib/knowledge_base/../../fixtures/parrot.png
```