Adjust to Rake 10.1.0

Because switch from Array to Scope for scope tracking.

See https://github.com/jimweirich/rake/commit/e063174394ce2278e0fb3f23f19c239207e869e5
Merge pull request #17 from myokoym/adjust-to-rake-10-1-0

Adjust to Rake 10.1.0

Patch by Masafumi Yokoyama. Thanks!!!