Use Gun master for tests

We no longer support OTP-19 so we don't need to stay on the
old Gun version anymore.

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 1.2.0
+dep_gun = git https://github.com/ninenines/gun master
 
 # CI configuration.