Override Bundler::GemHelper git push to work with boxen.

Rakefile

@@ -1 +1,16 @@
+module Bundler
+  class GemHelper
+    def perform_git_push_with_clean_env(options = '')
+      # Using a clean ENV ensures that ruby-based git credential helpers
+      # such as that used by boxen will still work:
+      Bundler.with_clean_env do
+        perform_git_push_without_clean_env(options)
+      end
+    end
+
+    alias_method :perform_git_push_without_clean_env, :perform_git_push
+    alias_method :perform_git_push, :perform_git_push_with_clean_env
+  end
+end
+
 require 'bundler/gem_tasks'