Adds an "Update RubyGems" step (again)

RubyGems has changed the security handling for connecting to the gem
server again. The version of RubyGems that ships with RailsInstaller
doesn't have the updated CA certificates, and so won't `gem install`
anything.

This commit adds some notes on how to fix this, based on similar notes
that were added earlier in the year for a different version of the same
problem.

Author: sarcas

sites/en/installfest/windows.step

@@ -82,6 +82,40 @@ step "Open a Terminal" do
   end
 end
 
+step "Update RubyGems" do
+  message "The version of RubyGems that comes with RailsInstaller has some problems. Follow these steps to upgrade it!"
+
+  step "Check to see if you need to update" do
+    console "gem -v"
+
+    message "If the output is **2.6.6** or earlier, keep following the instructions."
+
+    message "If the output is **2.6.7** or later, <a href='#install-node'>skip to the next step!</a>"
+  end
+
+  step "Download the update" do
+    message "Visit https://rubygems.org/downloads/rubygems-update-2.6.7.gem"
+
+    message "**Right click -> Save target as...** the file **rubygems-update-2.6.7.gem** to your **C:\\Sites** directory"
+  end
+
+  step "Install the update" do
+    message "Back at the command prompt, run the following commands:"
+
+    console_without_message "gem install --local C:\\Sites\\rubygems-update-2.6.7.gem"
+
+    console_without_message "update_rubygems --no-document"
+
+    message "**Close and reopen your command prompt**, then verify you have the upgraded RubyGems by typing this in the terminal:"
+
+    console_without_message "gem -v"
+    result "2.6.7"
+
+    message "Finally, you can clean up by running the following command:"
+    console_without_message "gem uninstall rubygems-update -x"
+  end
+end
+
 a name: 'install-node'
 step "Install Node.js" do