vendor: update gorilla/mux to be compatible with Go 1.7

Signed-off-by: Stephen J Day <stephen.day@docker.com>

vendor/github.com/gorilla/mux/context_gorilla.go

@@ -0,0 +1,26 @@
+// +build !go1.7
+
+package mux
+
+import (
+	"net/http"
+
+	"github.com/gorilla/context"
+)
+
+func contextGet(r *http.Request, key interface{}) interface{} {
+	return context.Get(r, key)
+}
+
+func contextSet(r *http.Request, key, val interface{}) *http.Request {
+	if val == nil {
+		return r
+	}
+
+	context.Set(r, key, val)
+	return r
+}
+
+func contextClear(r *http.Request) {
+	context.Clear(r)
+}