Put proper minitest plugin declaration in Rakefile

Now the unit tests pass on my Mac.

    $ rake test
    /usr/local/Cellar/ruby/1.9.3-p385/bin/ruby -w -Ilib:bin:test:. -e 'require "rubygems"; gem "minitest"; require "minitest/autorun"; require "test/test_hoe_test.rb"; require "test/test_hoe.rb"; require "test/test_hoe_debug.rb"; require "test/test_hoe_gemcutter.rb"; require "test/test_hoe_publish.rb"; require "test/test_hoe_test.rb"' -- 
    Run options: --seed 13050

    # Running tests:

    .............................

    Finished tests in 0.695548s, 41.6937 tests/s, 130.8321 assertions/s.

    29 tests, 91 assertions, 0 failures, 0 errors, 0 skips