title: gem list is showing two versions of a Ruby gem immediately after updating the gem
body: <p>I just updated the mime-types gem with <code>gem update mime-types</code>. <code>gem list</code> displayed <code>mime-types (1.16)</code> prior to updating. After the update <code>gem list</code> shows <code>mime-types (1.17.2, 1.16)</code>. Why are two version displayed?</p>
  
  <p>More info: I have other Rails projects on the same computer. I have not updated the mime-types gem in any other projects. Running <code>gem list</code> from another project's directory (where mime-types has not been updated) displays <code>mime-types (1.16)</code>.</p>