sublime_syntax_convertor: Add bundler updater to easily update gems.

pkgs/by-name/su/sublime_syntax_convertor/Gemfile.lock

···
 GEM
   remote: https://rubygems.org/
   specs:
-    plist (3.7.1)
+    plist (3.7.2)
     sublime_syntax_convertor (0.1.0)
       plist
 
 PLATFORMS
   ruby
-  x86_64-linux
 
 DEPENDENCIES
   sublime_syntax_convertor!
 
 BUNDLED WITH
-   2.5.5
+   2.6.6

pkgs/by-name/su/sublime_syntax_convertor/gemset.nix

···
     platforms = [ ];
     source = {
       remotes = [ "https://rubygems.org" ];
-      sha256 = "0b643i5b7b7galvlb2fc414ifmb78b5lsq47gnvhzl8m27dl559z";
+      sha256 = "0hlaf4b3d8grxm9fqbnam5gwd55wvghl0jyzjd1hc5hirhklaynk";
       type = "gem";
     };
-    version = "3.7.1";
+    version = "3.7.2";
   };
   sublime_syntax_convertor = {
     dependencies = [ "plist" ];

pkgs/by-name/su/sublime_syntax_convertor/package.nix

···
-{ lib, bundlerApp }:
-bundlerApp {
+{
+  lib,
+  bundlerApp,
+  bundlerUpdateScript,
+}:
+
+bundlerApp rec {
   pname = "sublime_syntax_convertor";
   gemdir = ./.;
   exes = [ "sublime_syntax_convertor" ];
+
+  passthru.updateScript = bundlerUpdateScript pname;
 
   meta = {
     description = "Converts tmLanguage to sublime-syntax";