(Bug 1035129) Fix missing job signatures

I changed the unique key on the reference data signatures table to
 (signature, name, build_system_type, repository). I also added the
device_name information to the payload submitted by
fetch_buildapi_pending and fetch_buildapi_running. That was the main
cause for the repeated signatures in the reference data signatures
table.
Merge pull request #190 from mozilla/bug-1035129-missing-job-signatures

(Bug 1035129) Fix missing job signatures