roles/nginx: Always set HSTS header

nginx 1.7.5 allows us to always set HTTP headers:

See: http://mailman.nginx.org/pipermail/nginx-announce/2014/000145.html

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

roles/nginx/templates/https.j2

@@ -32,5 +32,5 @@
     spdy_headers_comp 6;
 
     # Enable this if you want HSTS (recommended, but be careful)
-    add_header Strict-Transport-Security max-age=15768000;
+    add_header Strict-Transport-Security max-age=15768000 always;