Fix different MSI builds with same version not triggering removal of existing installation (beeware#99)

Author: abdnh

{{ cookiecutter.format }}/{{ cookiecutter.app_name }}.wxs

@@ -16,6 +16,11 @@
 
         <Media Id="1" Cabinet="product.cab" EmbedCab="yes" />
 
+        <MajorUpgrade
+            DowngradeErrorMessage="A newer version of [ProductName] is already installed."
+            AllowSameVersionUpgrades="yes"
+            Schedule="afterInstallInitialize" />
+
         <Icon Id="ProductIcon" SourceFile="icon.ico" />
 
         <!-- Add/Remove Programs settings -->