Fix changelog order in RPM generator

It looks like I misunderstood `{}.get(k)`, and thought it was sorting by key or something. In any case, the current behavior sorts by the date *alphabetically*, which is obviously wrong. This throws an error in SRPM build. Instead of sorting by version number (which I believe was my original intent), we should sort by date. The SRPM build cares more that the changelog is chronological by date, less so about the versions associated with each changelog entry.