Fix build start time retrieving

When build is not finished getTimes() returns tuple (start_time, None),
and min((start_time, None)) == None.  This lead to incorrect sorting of
builds in grid view --- not finished builds were displayed first and if
there were more build revisions than grid width builds (five by default)
not finished builds doesn't displayed at all.
Merge branch 'grid_build_time_bug' of git://github.com/vrutsky/buildbot