require golang version 1.11+
require golang version 1.11+

See also PR 4470 and commit dba155b8f5e5
which introduced a dependency on golang 1.10+.
Some docs refer to 1.11+ so I made bootstrap.sh require that.

Signed-off-by: Scott Lanning <55746cdfc34815fb7cc2d5c71b63445a84ea9920@booking.com>

dcadevil/vitess,davygeek/vitess,enisoc/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,dcadevil/vitess,mahak/vitess,tinyspeck/vitess,tinyspeck/vitess,HubSpot/vitess,dcadevil/vitess,mahak/vitess,vitessio/vitess,HubSpot/vitess,dcadevil/vitess,davygeek/vitess,enisoc/vitess,tinyspeck/vitess,enisoc/vitess,vitessio/vitess,enisoc/vitess,HubSpot/vitess,vitessio/vitess,enisoc/vitess,davygeek/vitess,dcadevil/vitess,mahak/vitess,HubSpot/vitess,tinyspeck/vitess,dcadevil/vitess,davygeek/vitess,HubSpot/vitess,dcadevil/vitess,davygeek/vitess,HubSpot/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,davygeek/vitess,vitessio/vitess,enisoc/vitess,davygeek/vitess,tinyspeck/vitess,mahak/vitess,mahak/vitess,HubSpot/vitess,vitessio/vitess,HubSpot/vitess,enisoc/vitess