Upgrade ruby bundler used in sample app

Do not fail if we restore a previous layer containing a `bundler`
directory.

apps/ruby-bundler/Gemfile.lock

@@ -21,4 +21,4 @@ DEPENDENCIES
   sinatra
 
 BUNDLED WITH
-   2.0.2
+   2.3.26

buildpacks/ruby-bundler/bin/build

@@ -43,7 +43,7 @@ if [[ -f Gemfile.lock && ${local_bundler_checksum} == ${remote_bundler_checksum}
 else
   # Determine if there has been a gem dependency change and install new gems to the bundler layer; re-using existing and un-changed gems
   echo "---> Installing gems"
-  mkdir "${bundler_layer_dir}"
+  mkdir -p "${bundler_layer_dir}"
   cat > "${bundler_layer_dir}.toml" <<EOL
 cache = true
 launch = true