[travis] disable arm-gcc-8 until compiler bug is fixed (#4306)
[travis] disable arm-gcc-8 until compiler bug is fixed (#4306)


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