Use OkHttp to avoid connection issues in some android versions

see: https://github.com/slapperwan/gh4a/issues/181

The Issue: http://stackoverflow.com/questions/11810447/httpurlconnection-worked-fine-in-android-2-x-but-not-in-4-1-no-authentication-c

Solution: use http://square.github.io/okhttp/ . Their implementation correctly handles this issue