Add bionic as supported distro

Add the upcoming release Ubuntu 18.04 Bionic Beaver to the list of
supported distros. Drop the now unsupported 17.04 (zesty) instead.

Change-Id: Iea0b4bfdc510797f7886fac96eff6fdfb730252d
diff --git a/stack.sh b/stack.sh
index badc1a1..ce6e6fe 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|zesty|artful|stretch|jessie|f25|f26|f27|opensuse-42.3|opensuse-tumbleweed|rhel7) ]]; then
+if [[ ! ${DISTRO} =~ (xenial|artful|bionic|stretch|jessie|f25|f26|f27|opensuse-42.3|opensuse-tumbleweed|rhel7) ]]; 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"