title: gem install/update fails with "no implicit conversion of nil into String"
body: <p>Using rvm, anytime I try to install or update a gem (or rubygems), I get the same error:</p>
  
  <pre><code>$ gem update --system
  Updating rubygems-update
  Fetching: rubygems-update-2.6.6.gem (100%)
  ERROR:  While executing gem ... (TypeError)
      no implicit conversion of nil into String
  </code></pre>
  
  <p>My system ruby however, works fine.</p>