Turns out that str(object) is used when putting 'object' in the
template context. Because of revisions __unicode__ was using series.name
we were doing as many queries as revisions in the detailed series page,
without actually using that string representation anywhere.

Because the string representation of SeriesRevision is not very useful,
let's just get rid of that query altogether.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>