Use proxy_redirect to fix Location headers

When SSL is enabled, Jenkins displays the following error message on the Manage Jenkins page:

    It appears that your reverse proxy set up is broken.

This pull request fixes this issue using nginx's `proxy_redirect` command.  Nignx will change  rewrite redirects from Jenkins to the proper https scheme.

If opscode-cookbooks/jenkins#21 is ever landed, it would be really nice if it included this commit.