scripts: Purge old deployments on every upgrade.

Fixes: #554.

scripts/deploy-zulip-from-git

@@ -79,7 +79,5 @@ try:
 
     subprocess.check_call(["sudo", os.path.join(deploy_path, "scripts", "lib", "upgrade-zulip-stage-2"),
                            deploy_path, "--from-git"] + deploy_options)
-
-    subprocess.check_call(["./scripts/purge-old-deployments"])
 finally:
     release_deployment_lock()

scripts/lib/upgrade-zulip-stage-2

@@ -79,3 +79,5 @@ if os.path.exists("/etc/supervisor/conf.d/zulip_db.conf"):
    subprocess.check_call(["supervisorctl", "start", "process-fts-updates"], preexec_fn=su_to_zulip)
 
 logging.info("Upgrade complete!")
+
+subprocess.check_call(["./scripts/purge-old-deployments"])