Remove Ubuntu 14.10 as supported distribution

Ubuntu 14.10(utopic) reached end of life[1].

[1]: https://lists.ubuntu.com/archives/ubuntu-announce/2015-July/000197.html

Change-Id: Iab13ca797bda56462d9d117aa500d3ba0d9bebcb
diff --git a/stack.sh b/stack.sh
index 8024731..bdbb025 100755
--- a/stack.sh
+++ b/stack.sh
@@ -178,7 +178,7 @@
 
 # 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} =~ (precise|trusty|utopic|vivid|7.0|wheezy|sid|testing|jessie|f21|f22|rhel7) ]]; then
+if [[ ! ${DISTRO} =~ (precise|trusty|vivid|7.0|wheezy|sid|testing|jessie|f21|f22|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"