Use Github v3 API search_issues method

## Motivation
The legacy_search_issues method was deprecated in the Github v3 API. Today I noticed in logstash that I occasionally get 404s when using the legacy_search_issues method. It seems kind of silly to rely on a deprecated API call, when there's a replacement.

## Preliminary discussion
I did search the repository for anything related to legacy_search_issues, and found this PR: dockyard/party_foul#71
I have none of the problems listed there as I guess Octokit now works well with the v3 API.

## Summary of PR

I've updated the PartyFoul::ExceptionHandler#find_first_issue method to use the search_issues method rather than the legacy_search_issues. Then I fixed all legacy_search_issues tests and ran rake test with great success.

Also, just testing out github selfies :-)

![selfie-2](http://i.imgur.com/CJHN8Og.png)