Filter out invalid pull requests
Filter out invalid pull requests

According to [this
tweet](https://twitter.com/digitalocean/status/783763569812832256), pull
requests will not be counted toward Hacktoberfest if they are labelled
as "invalid". This change excludes those issues from the search results.

spezzino/hfchecker,spezzino/hfchecker,spezzino/hfchecker