minor: removing compiled c-extensions with test cleanup

Feel free to shoot this down if you guys thing its a bad idea, but I'd like to add `lib/bson_ext` to the list of directories we cleanup after each test run when `test:cleanup` is triggered. Right now, those compiled extensions hang around and it causes issues when you're testing across multiple rubies.

For example, I often like to do this:

```bash
rvm jruby,1.8.7,1.9.3,2.0.0 do bundle exec rake test:commit
```

However, if I'm testing with extensions enabled and switching between versions (especially for 1.8 to 2.0) this currently fails if those compiled c extensions stick around.