taskjuggler: add webrick for tj3webd

Without webrick, the taskjuggler web server fails to start.

Made by adding "gem 'webrick'" to the Gemfile and running
`nix-shell maintainers/scripts/update.nix --argstr package taskjuggler`.

pkgs/applications/misc/taskjuggler/Gemfile

···
 source 'https://rubygems.org'
 gem 'taskjuggler'
+gem 'webrick'

pkgs/applications/misc/taskjuggler/Gemfile.lock

···
     timeout (0.3.2)
     tins (1.32.1)
       sync
+    webrick (1.8.1)
 
 PLATFORMS
-  ruby
+  x86_64-linux
 
 DEPENDENCIES
   taskjuggler
+  webrick
 
 BUNDLED WITH
    2.4.12

pkgs/applications/misc/taskjuggler/gemset.nix

···
     };
     version = "1.32.1";
   };
+  webrick = {
+    groups = ["default"];
+    platforms = [];
+    source = {
+      remotes = ["https://rubygems.org"];
+      sha256 = "13qm7s0gr2pmfcl7dxrmq38asaza4w0i2n9my4yzs499j731wh8r";
+      type = "gem";
+    };
+    version = "1.8.1";
+  };
 }