test: Move "unit_race" test from Travis shard 4 to 0.
test: Move "unit_race" test from Travis shard 4 to 0.

This will balance the shards better. This test in particular takes ~3m48s.

Exampe for Shard Execution times before this change:
(see https://travis-ci.org/youtube/vitess/builds/159687311)

shard 0:  6m55s
shard 1: 11m48s
shard 2: 12m08s
shard 3: 14m46s
shard 4: 17m20s

Eventually, we could do the automatic rebalancing again :)

dcadevil/vitess,dcadevil/vitess,HubSpot/vitess,mahak/vitess,mapbased/vitess,mattharden/vitess,tirsen/vitess,rnavarro/vitess,alainjobart/vitess,mahak/vitess,davygeek/vitess,tinyspeck/vitess,sougou/vitess,dumbunny/vitess,mapbased/vitess,alainjobart/vitess,mattharden/vitess,mahak/vitess,HubSpot/vitess,mapbased/vitess,davygeek/vitess,mattharden/vitess,dcadevil/vitess,tinyspeck/vitess,pivanof/vitess,sougou/vitess,sougou/vitess,pivanof/vitess,dcadevil/vitess,mattharden/vitess,tirsen/vitess,sougou/vitess,pivanof/vitess,tirsen/vitess,vitessio/vitess,rnavarro/vitess,dumbunny/vitess,alainjobart/vitess,davygeek/vitess,pivanof/vitess,alainjobart/vitess,dumbunny/vitess,tirsen/vitess,sougou/vitess,mattharden/vitess,tinyspeck/vitess,dcadevil/vitess,applift/vitess,tinyspeck/vitess,vitessio/vitess,pivanof/vitess,enisoc/vitess,enisoc/vitess,HubSpot/vitess,vitessio/vitess,applift/vitess,pivanof/vitess,tirsen/vitess,pivanof/vitess,dumbunny/vitess,mattharden/vitess,pivanof/vitess,davygeek/vitess,mapbased/vitess,mahak/vitess,mattharden/vitess,tinyspeck/vitess,vitessio/vitess,tirsen/vitess,vitessio/vitess,applift/vitess,vitessio/vitess,rnavarro/vitess,mattharden/vitess,rnavarro/vitess,applift/vitess,applift/vitess,dcadevil/vitess,mahak/vitess,HubSpot/vitess,mahak/vitess,sougou/vitess,applift/vitess,davygeek/vitess,tinyspeck/vitess,HubSpot/vitess,HubSpot/vitess,applift/vitess,enisoc/vitess,alainjobart/vitess,applift/vitess,dcadevil/vitess,tirsen/vitess,HubSpot/vitess,dumbunny/vitess,tirsen/vitess,mapbased/vitess,mattharden/vitess,dumbunny/vitess,enisoc/vitess,davygeek/vitess,tinyspeck/vitess,applift/vitess,vitessio/vitess,alainjobart/vitess,enisoc/vitess,rnavarro/vitess,mapbased/vitess,vitessio/vitess,rnavarro/vitess,sougou/vitess,applift/vitess,mapbased/vitess,HubSpot/vitess,mahak/vitess,mapbased/vitess,enisoc/vitess,rnavarro/vitess,dumbunny/vitess,alainjobart/vitess,mattharden/vitess,sougou/vitess,alainjobart/vitess,pivanof/vitess,rnavarro/vitess,mapbased/vitess,dumbunny/vitess,mapbased/vitess,davygeek/vitess,enisoc/vitess,dumbunny/vitess,dumbunny/vitess,mahak/vitess,rnavarro/vitess,tirsen/vitess