Update module gorilla/mux to v1.7.4

Author: renovate-bot

go.mod

@@ -28,7 +28,7 @@ require (
 	github.com/gomodule/redigo v2.0.0+incompatible // indirect
 	github.com/google/go-github v0.0.0-20180123235826-b1f138353a62
 	github.com/gopherjs/gopherjs v0.0.0-20190430165422-3e4dfb77656c // indirect
-	github.com/gorilla/mux v1.7.3
+	github.com/gorilla/mux v1.7.4
 	github.com/gorilla/schema v1.1.0
 	github.com/gorilla/securecookie v1.1.1
 	github.com/gorilla/sessions v1.1.3