[travis] add GCC 9 to build matrix (#4055)
[travis] add GCC 9 to build matrix (#4055)


librasungirl/openthread,abtink/openthread,srickardti/openthread,bukepo/openthread,chshu/openthread,turon/openthread,librasungirl/openthread,librasungirl/openthread,srickardti/openthread,lanyuwen/openthread,jwhui/openthread,abtink/openthread,turon/openthread,srickardti/openthread,chshu/openthread,turon/openthread,lanyuwen/openthread,lanyuwen/openthread,jwhui/openthread,lanyuwen/openthread,openthread/openthread,openthread/openthread,chshu/openthread,lanyuwen/openthread,srickardti/openthread,librasungirl/openthread,openthread/openthread,chshu/openthread,jwhui/openthread,bukepo/openthread,abtink/openthread,openthread/openthread,chshu/openthread,abtink/openthread,turon/openthread,bukepo/openthread,lanyuwen/openthread,bukepo/openthread,chshu/openthread,jwhui/openthread