title: Rake doesn't know how to build tasks: default or minitest
body: <p>I built a gem a while back and did not include any tests (shame on me). I've since attempted to redress this by including minitest, but I'm getting stuck right out of the gate with the following error:</p>
  
  <blockquote>
    <p>Don't know how to build task 'default'</p>
  </blockquote>
  
  <p>Here's my <strong>Rakefile</strong></p>
  
  <pre><code>require 'rake/testtask'
  
  Rake::TestTask.new do |t|
    t.test_files = FileList['test/*_test.rb']
  end
  </code></pre>
  
  <p>And the dummy file in <code>test/unit_test.rb</code></p>
  
  <pre><code>require 'minitest/autorun'
  
  class TestPackage &lt; MiniTest::Unit::TestCase
  
    def test
      assert_equal 10, Array.new(10).size
    end
  end
  </code></pre>
  
  <p>If I change the Rakefile to include <code>task :default =&gt; 'minitest'</code> the error message changes to <code>Don't know how to build task 'minitest'</code></p>
  
  <p><code>rake --trace</code> seems to just direct me back to the parts of the Rake gem that handle task invocation. No clues that I can see:</p>
  
  <pre><code>~/.rbenv/versions/1.9.3-p286/lib/ruby/1.9.1/rake/task_manager.rb:49:in `[]'
  ~/.rbenv/versions/1.9.3-p286/lib/ruby/1.9.1/rake/application.rb:115:in `invoke_task'
  ~/.rbenv/versions/1.9.3-p286/lib/ruby/1.9.1/rake/application.rb:94:in `block (2 levels) in top_level'
  ~/.rbenv/versions/1.9.3-p286/lib/ruby/1.9.1/rake/application.rb:94:in `each'
  ~/.rbenv/versions/1.9.3-p286/lib/ruby/1.9.1/rake/application.rb:94:in `block in top_level'
  ~/.rbenv/versions/1.9.3-p286/lib/ruby/1.9.1/rake/application.rb:133:in `standard_exception_handling'
  ~/.rbenv/versions/1.9.3-p286/lib/ruby/1.9.1/rake/application.rb:88:in `top_level'
  ~/.rbenv/versions/1.9.3-p286/lib/ruby/1.9.1/rake/application.rb:66:in `block in run'
  ~/.rbenv/versions/1.9.3-p286/lib/ruby/1.9.1/rake/application.rb:133:in `standard_exception_handling'
  ~/.rbenv/versions/1.9.3-p286/lib/ruby/1.9.1/rake/application.rb:63:in `run'
  ~/.rbenv/versions/1.9.3-p286/bin/rake:32:in `&lt;main&gt;'
  </code></pre>