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!!!
Specify encoding when reading external file

Because the following error "invalid byte sequence" occurred.

    % git clone https://github.com/rcairo/rcairo
    % cd rcairo
    % LANG=C rake -T
    rake aborted!
    There was a ArgumentError while loading cairo.gemspec:
    invalid byte sequence in US-ASCII from
    ...