Convert ListImages to use reauth.

This commit does not work -- acceptance test 08-... is broken.
diff --git a/context.go b/context.go
index 2c70ba5..ebad36c 100644
--- a/context.go
+++ b/context.go
@@ -49,6 +49,9 @@
 	return &Context{
 		providerMap: make(map[string]Provider),
 		httpClient:  &http.Client{},
+		reauthHandler: func(acc AccessProvider) error {
+			return acc.Reauthenticate()
+		},
 	}
 }