Add a dependency on postgresql.service to xsnippet-api.service

xsnippet-api tries to connect to PostgreSQL on start and will fail
if it's not available. Declare this dependency in systemd, so that
the services always come up in the right order.

roles/xsnippet_api/templates/systemd.service.j2

@@ -1,7 +1,7 @@
 [Unit]
 Description = XSnippet API
-After = network.target network-online.target
-Wants = network-online.target
+After = network.target network-online.target postgresql.service
+Wants = network-online.target postgresql.service
 StartLimitIntervalSec = 0
 
 [Service]