roles/nginx: Correct HSTS header in https template

Apparently the "always" syntax isn't used anymore (ever?), not sure
where I got it from but this definitely causes HSTS to not work.

See: https://mozilla.github.io/server-side-tls/ssl-config-generator/
See: https://raymii.org/s/tutorials/HTTP_Strict_Transport_Security_for_Apache_NGINX_and_Lighttpd.html

Signed-off-by: Alan Orth <alan.orth@gmail.com>

roles/nginx/templates/https.j2

@@ -35,5 +35,5 @@
     spdy_headers_comp 6;
 
     # Enable this if you want HSTS (recommended, but be careful)
-    add_header Strict-Transport-Security max-age=15768000 always;
+    add_header Strict-Transport-Security max-age=15768000;