Updated tracker core version
Check for a HttpResponse if it returns null

- We now check if we don't get a response before we try to retrieve a
  status code from it. This fixes a serious issue with the tracker
  crashing on network failure attempts.
- Fixes #32
- Closes #33 since the update to a newer version is already done
Update CHANGELOG
Updated completePayload to take timestamp as a long (closes #35)
Updated auto-generated Version