Avoid use of negative checks for directories

When checking for the existence of directories, do not use
the negative expression as it makes the code less readable.

Change-Id: Ic00b819476bff6cba59bf1a8b39f2f2b2af55c71
diff --git a/fpm/scripts/after-install.sh b/fpm/scripts/after-install.sh
index 89780c6..f6e0f49 100755
--- a/fpm/scripts/after-install.sh
+++ b/fpm/scripts/after-install.sh
@@ -2,7 +2,7 @@
 LOG=/tmp/gerrit-installer.log
 chmod a+rw $LOG
 
-[ ! -d /etc/default ] && mkdir -p /etc/default
+[ -d /etc/default ] || mkdir -p /etc/default
 echo "GERRIT_SITE=$GERRIT_SITE" > /etc/default/gerritcodereview
 
 echo ""