[travis] switch to ubuntu bionic (#4222)
[travis] switch to ubuntu bionic (#4222)


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