poky.conf: bump version for 3.1.19 release

(From meta-yocto rev: 2de758bc8a4ead8e89619766d5096604b554f2c1)

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-poky/conf/distro/poky.conf

 DISTRO = "poky"
 DISTRO_NAME = "Poky (Yocto Project Reference Distro)"
-DISTRO_VERSION = "3.1.18"
+DISTRO_VERSION = "3.1.19"
 DISTRO_CODENAME = "dunfell"
 SDK_VENDOR = "-pokysdk"
 SDK_VERSION = "${@d.getVar('DISTRO_VERSION').replace('snapshot-${DATE}', 'snapshot')}"