Use Gun 1.3.0 instead of master for tests

Makefile

@@ -22,7 +22,7 @@ DOC_DEPS = asciideck
 
 TEST_DEPS = $(if $(CI_ERLANG_MK),ci.erlang.mk) ct_helper gun proper
 dep_ct_helper = git https://github.com/extend/ct_helper master
-dep_gun = git https://github.com/ninenines/gun master
+dep_gun = git https://github.com/ninenines/gun 1.3.0
 
 # CI configuration.