Escape variable in nginx post-deploy script

`$http_upgrade` is supposed to be a variable set by nginx. It was now substituted by nothing.

plugins/nginx-vhosts/post-deploy

@@ -12,11 +12,11 @@ server {
   location    / {
     proxy_pass  http://$APP;
     proxy_http_version 1.1;
-    proxy_set_header Upgrade $http_upgrade;
+    proxy_set_header Upgrade \$http_upgrade;
     proxy_set_header Connection "upgrade"; 
   }
 }
 EOF
   nc -U $HOME/reload-nginx
   echo "$hostname" > "$HOME/$APP/VHOST"
-fi
+fi