title: How can I change the default minitest version
body: <p>After install ruby 2.1.2 on my Macbook Pro, I found the default minitest version is <code>4.7.5</code>, I do <code>sodo gem install minitest</code> to install minitest <code>5.4.2</code>, but the default gem is <code>4.7.5</code> yet.  I also do <code>gem cleanup</code>,that didn't remove the old version.I had try <code>gem uninstall minitest -v 4.7.5</code> and got </p>
  
  <p><code>
  RROR:  While executing gem ... (Gem::InstallError)
      gem "minitest" cannot be uninstalled because it is a default gem
  </code></p>
  
  <p>So how can I change the default gem to version 5.4.2 and remove the old one? 
  If I don't change this, I have to write  <code>class AbcTest &lt; MiniTest::Unit::TestCase</code> not <code>class AbcTest &lt; Minitest::Test</code> in my test. Or modify Gemfile everytime I create a new project.</p>
  
  <p>PS:</p>
  
  <p><strong>Ruby version:</strong> <code>ruby 2.1.2p95 (2014-05-08 revision 45877) [x86_64-darwin13.0]</code></p>
  
  <p><strong>gem list minitest -d</strong> </p>
  
  <p><code>minitest (5.4.2, 4.7.5)
      Author: Ryan Davis
      Homepage: https://github.com/seattlerb/minitest
      License: MIT
      Installed at (5.4.2): /Users/xxxxx/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0
                   (4.7.5, default): /Users/xxxxx/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0
      minitest provides a complete suite of testing facilities supporting
      TDD, BDD, mocking, and benchmarking
  </code></p>