PR links point to https://null/...

Using v0.0.11 and the following command:

```
github-changes -o mikeal -r request -a -v -f CHANGELOG.md 
```

I get content like this:

```
### upcoming (2014/07/09 12:01 +00:00)
  4 - [#946](https://null/mikeal/request/pull/946) Merge pull request #946 from aj0strow/merge-default-headers (@aj0strow)
  5 - [#844](https://null/mikeal/request/pull/844) Merge pull request #844 from jvmccarthy/issue595_http_proxy_env_vars (@jvmccarthy)
```