Make regs into optional object instead of list

Was getting parsing errors complaining about expecting an array for regs.

Example of regs object in request:
"regs":{"coppa":1}

--------------------------[Exception thrown]---------------------------
time:   2014-09-04T19:12:43Z
type:   Datacratic::StructureDescriptionBase::Exception
pid:    11094; tid: 11243
what:   at .regs: Bid Request:1:859: at .regs: expected array of OpenRTB::Regulations