REGTEST: make agent-check.vtc require 1.8

This one looks at the output of "show server state", which change slightly
in 1.8 (shows DNS fields).
diff --git a/reg-tests/checks/agent-check.vtc b/reg-tests/checks/agent-check.vtc
index eb2e08e..1345693 100644
--- a/reg-tests/checks/agent-check.vtc
+++ b/reg-tests/checks/agent-check.vtc
@@ -1,4 +1,5 @@
 varnishtest "Health-checks: agent-check"
+#REQUIRE_VERSION=1.8
 #REGTEST_TYPE=slow
 feature ignore_unknown_macro