Watch out for missing rels in Link header.

Fix for https://github.com/Strider-CD/strider/issues/413.

GitHub is sending Link headers without rel="first" or rel="next" which crashes Strider when you try to refresh your GitHub projects.

The Link header I logged right before the crash only had a rel="last". I don't know if that's just a temporary mistake on GitHub's part, but removing the check for the first link like I did in this pull request prevents Strider from crashing.

I haven't seen any other errors since I made this patch, but I'm new to Strider so might be missing a lot.