title: Ruby on Rails: can't update with gem update
body: <p>Running <code>gem -v</code> gives:</p>
  
  <pre><code>1.8.11
  </code></pre>
  
  <p>Running <code>gem update</code>:</p>
  
  <pre><code>Updating installed gems
  Updating actionmailer
  Fetching: activesupport-3.1.3.gem (100%)
  Fetching: activemodel-3.1.3.gem (100%)
  Fetching: sprockets-2.0.3.gem (100%)
  Fetching: actionpack-3.1.3.gem (100%)
  Fetching: actionmailer-3.1.3.gem (100%)
  Successfully installed activesupport-3.1.3
  Successfully installed activemodel-3.1.3
  Successfully installed sprockets-2.0.3
  Successfully installed actionpack-3.1.3
  Successfully installed actionmailer-3.1.3
  Updating activerecord
  Fetching: activerecord-3.1.3.gem (100%)
  Successfully installed activerecord-3.1.3
  Updating activeresource
  Fetching: activeresource-3.1.3.gem (100%)
  Successfully installed activeresource-3.1.3
  Updating childprocess
  Fetching: childprocess-0.2.3.gem (100%)
  Successfully installed childprocess-0.2.3
  Updating columnize
  Fetching: columnize-0.3.5.gem (100%)
  Successfully installed columnize-0.3.5
  Updating cucumber
  Fetching: gherkin-2.6.8.gem (100%)
  Building native extensions.  This could take a while...
  Fetching: cucumber-1.1.3.gem (100%)
  
  (::) (::) (::) (::) (::) (::) (::) (::) (::) (::) (::) (::) (::) (::) (::)
  
  Thank you for installing cucumber-1.1.3.
  Please be sure to read http://wiki.github.com/cucumber/cucumber/upgrading
  for important information about this release. Happy cuking!
  
  (::) (::) (::) (::) (::) (::) (::) (::) (::) (::) (::) (::) (::) (::) (::)
  
  Successfully installed gherkin-2.6.8
  Successfully installed cucumber-1.1.3
  Updating devise
  Fetching: devise-1.5.2.gem (100%)
  Successfully installed devise-1.5.2
  Updating factory_girl
  Fetching: factory_girl-2.3.2.gem (100%)
  Successfully installed factory_girl-2.3.2
  Updating jquery-rails
  Fetching: jquery-rails-1.0.19.gem (100%)
  Successfully installed jquery-rails-1.0.19
  Updating json
  Fetching: json-1.6.3.gem (100%)
  Building native extensions.  This could take a while...
  Successfully installed json-1.6.3
  Updating json_pure
  Fetching: json_pure-1.6.3.gem (100%)
  Successfully installed json_pure-1.6.3
  Updating multi_json
  Fetching: multi_json-1.0.4.gem (100%)
  Successfully installed multi_json-1.0.4
  Updating passenger
  Fetching: passenger-3.0.11.gem (100%)
  Successfully installed passenger-3.0.11
  Updating rails
  Fetching: railties-3.1.3.gem (100%)
  Fetching: rails-3.1.3.gem (100%)
  Successfully installed railties-3.1.3
  Successfully installed rails-3.1.3
  Updating rubyzip
  Fetching: rubyzip-0.9.5.gem (100%)
  Successfully installed rubyzip-0.9.5
  Updating sass
  Fetching: sass-3.1.11.gem (100%)
  Successfully installed sass-3.1.11
  Updating selenium-webdriver
  Fetching: selenium-webdriver-2.14.0.gem (100%)
  Successfully installed selenium-webdriver-2.14.0
  Gems updated: activesupport, activemodel, sprockets, actionpack, actionmailer, activerecord, activeresource, childprocess, columnize, gherkin, cucumber, devise, factory_girl, jquery-rails, json, json_pure, multi_json, passenger, railties, rails, rubyzip, sass, selenium-webdriver
  Installing ri documentation for activesupport-3.1.3...
  Installing ri documentation for activemodel-3.1.3...
  Installing ri documentation for sprockets-2.0.3...
  Installing ri documentation for actionpack-3.1.3...
  Installing ri documentation for actionmailer-3.1.3...
  Installing ri documentation for activerecord-3.1.3...
  Installing ri documentation for activeresource-3.1.3...
  Installing ri documentation for childprocess-0.2.3...
  Installing ri documentation for columnize-0.3.5...
  Installing ri documentation for gherkin-2.6.8...
  Installing ri documentation for cucumber-1.1.3...
  Installing ri documentation for devise-1.5.2...
  Installing ri documentation for factory_girl-2.3.2...
  Installing ri documentation for jquery-rails-1.0.19...
  Installing ri documentation for json-1.6.3...
  Installing ri documentation for json_pure-1.6.3...
  Installing ri documentation for multi_json-1.0.4...
  Installing ri documentation for passenger-3.0.11...
  Installing ri documentation for railties-3.1.3...
  Installing ri documentation for rails-3.1.3...
  Installing ri documentation for rubyzip-0.9.5...
  Installing ri documentation for sass-3.1.11...
  Installing ri documentation for selenium-webdriver-2.14.0...
  ERROR:  While executing gem ... (ArgumentError)
      marshal data too short
  </code></pre>
  
  <p>What should i do ?</p>