Use released fork of github-api

I've published a small fork of the github-api, which includes these
pull-requests we need:

https://github.com/kohsuke/github-api/pull/66 - org member paging
https://github.com/kohsuke/github-api/pull/68 - org-member filtering
Merge pull request #6 from guardian/use-released-fork-of-github-api

Use released fork of github-api