Fix Shellcheck SC2086: Quote to prevent splitting

Double quote to prevent globbing and word splitting.

https://github.com/koalaman/shellcheck/wiki/SC2086

Signed-off-by: Dan Callahan <danc@element.io>

demo/clean.sh

@@ -6,14 +6,14 @@ DIR="$( cd "$( dirname "$0" )" && pwd )"
 
 PID_FILE="$DIR/servers.pid"
 
-if [ -f $PID_FILE ]; then
+if [ -f "$PID_FILE" ]; then
     echo "servers.pid exists!"
     exit 1
 fi
 
 for port in 8080 8081 8082; do
-    rm -rf ${DIR:?}/$port
-    rm -rf $DIR/media_store.$port
+    rm -rf "${DIR:?}/$port"
+    rm -rf "$DIR/media_store.$port"
 done
 
-rm -rf ${DIR:?}/etc
+rm -rf "${DIR:?}/etc"