(bug 1072291) revert pushlog caching strategy

The pushlog cache now uses the top revision of the last push.
Also, increase the time limit to fetch the pushlog to 3 minutes
Bug 1072291 - handle 404 responses from json-pushes
Merge pull request #229 from mozilla/bug-1072291-fix-pushlog-ingestion

(bug 1072291) revert pushlog caching strategy