List openSUSE 42.3 as a supported distro

openSUSE 42.3 is a relatively minor update over 42.2 and I'd
like to maintain it and keep it passing.

Experimental gates are being proposed, once those are passing
(which should be the case with this patch included), we could
enable it as a non-voting gate.

Change-Id: Ia421ada0ed3751c65a2a93a208e3f4a43edf8b16
diff --git a/stack.sh b/stack.sh
index c453b91..b786f7b 100755
--- a/stack.sh
+++ b/stack.sh
@@ -221,7 +221,7 @@
 
 # Warn users who aren't on an explicitly supported distro, but allow them to
 # override check and attempt installation with ``FORCE=yes ./stack``
-if [[ ! ${DISTRO} =~ (xenial|yakkety|zesty|stretch|jessie|f24|f25|opensuse-42.2|rhel7|kvmibm1) ]]; then
+if [[ ! ${DISTRO} =~ (xenial|yakkety|zesty|stretch|jessie|f24|f25|opensuse-42.2|opensuse-42.3|rhel7|kvmibm1) ]]; then
     echo "WARNING: this script has not been tested on $DISTRO"
     if [[ "$FORCE" != "yes" ]]; then
         die $LINENO "If you wish to run this script anyway run with FORCE=yes"