REGTEST: make abns_socket.vtc require 1.8

It's not as much an abns test as it is a seamless reload test, which
appeared in 1.8 due to "expose-fd listeners".
diff --git a/reg-tests/seamless-reload/abns_socket.vtc b/reg-tests/seamless-reload/abns_socket.vtc
index 7387760..3542948 100644
--- a/reg-tests/seamless-reload/abns_socket.vtc
+++ b/reg-tests/seamless-reload/abns_socket.vtc
@@ -13,6 +13,7 @@
 
 # abns@ sockets are not available on freebsd
 #EXCLUDE_TARGETS=freebsd,osx,generic
+#REQUIRE_VERSION=1.8
 #REGTEST_TYPE=bug
 
 haproxy h1 -W -conf {