title: Can't run rails server after accidentally running $ gem update --system
body: <p>I ran <code>gem update --system</code> thinking it would solve a problem I was having, and now I can't run <code>rails server</code>. It won't load and instead, I keep getting the following. Can someone tell me how to undo this and revert back to before I ran the update?</p>
  
  <p>I've already ran <code>git checkout</code>, but the problem persists...</p>
  
  <pre><code>/usr/local/rvm/gems/ruby-1.9.3-p194@global/gems/bundler-1.1.5/lib/bundler/rubygems_integration.rb:187:in `stub_source_index170': uninitialized constant Gem::SourceIndex (NameError)
  from /usr/local/rvm/gems/ruby-1.9.3-p194@global/gems/bundler-1.1.5/lib/bundler/rubygems_integration.rb:353:in `stub_rubygems'
  from /usr/local/rvm/gems/ruby-1.9.3-p194@global/gems/bundler-1.1.5/lib/bundler/rubygems_integration.rb:250:in `replace_entrypoints'
  from /usr/local/rvm/gems/ruby-1.9.3-p194@global/gems/bundler-1.1.5/lib/bundler/runtime.rb:14:in `setup'
  from /usr/local/rvm/gems/ruby-1.9.3-p194@global/gems/bundler-1.1.5/lib/bundler.rb:107:in `setup'
  from /usr/local/rvm/gems/ruby-1.9.3-p194@global/gems/rubygems-bundler-1.0.7/lib/rubygems-bundler/noexec.rb:66:in `setup'
  from /usr/local/rvm/gems/ruby-1.9.3-p194@global/gems/rubygems-bundler-1.0.7/lib/rubygems-bundler/noexec.rb:78:in `&lt;top (required)&gt;'
  from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:110:in `require'
  from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:110:in `rescue in require'
  from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:35:in `require'
  from /usr/local/rvm/gems/ruby-1.9.3-p194/bin/ruby_noexec_wrapper:9:in `&lt;main&gt;'
  </code></pre>