vagrant: mark bundler as system plugin

This will cause Vagrant to use the rubygems version of bundler
without complaint.

pkgs/development/tools/vagrant/default.nix

@@ -38,6 +38,7 @@ in buildRubyGem rec {
 
   patches = [
     ./unofficial-installation-nowarn.patch
+    ./use-system-bundler-version.patch
   ];
 
   # PATH additions:

pkgs/development/tools/vagrant/use-system-bundler-version.patch

@@ -0,0 +1,13 @@
+diff --git i/lib/vagrant/bundler.rb w/lib/vagrant/bundler.rb
+index 301e40e37..e361ab510 100644
+--- i/lib/vagrant/bundler.rb
++++ w/lib/vagrant/bundler.rb
+@@ -217,7 +217,7 @@ module Vagrant
+       source_list = {}
+       system_plugins = plugins.map do |plugin_name, plugin_info|
+         plugin_name if plugin_info["system"]
+-      end.compact
++      end.compact << "bundler"
+       installer_set = VagrantSet.new(:both)
+       installer_set.system_plugins = system_plugins
+