Update default.rb

It seems when i attempted to upload the cookbook, and ran it it asked for the jenkins name.  i believe it defaults to chef-jenkins so heres the fix :)

```
[2013-06-17T18:49:41+00:00] WARN: Cloning resource attributes for execute[apt-get update] from prior resource (CHEF-3694)
[2013-06-17T18:49:41+00:00] WARN: Previous execute[apt-get update]: /var/chef/cache/cookbooks/ops/recipes/packages.rb:12:in `from_file'
[2013-06-17T18:49:41+00:00] WARN: Current  execute[apt-get update]: /var/chef/cache/cookbooks/apt/recipes/default.rb:29:in `from_file'

================================================================================
Recipe Compile Error in /var/chef/cache/cookbooks/chef-jenkins/recipes/default.rb
================================================================================


Chef::Exceptions::CookbookNotFound
----------------------------------
Cookbook jenkins not found. If you're loading jenkins from another cookbook, make sure you configure the dependency in your metadata


Cookbook Trace:
---------------
  /var/chef/cache/cookbooks/chef-jenkins/recipes/default.rb:256:in `from_file'


Relevant File Content:
----------------------
/var/chef/cache/cookbooks/chef-jenkins/recipes/default.rb:

249:  end
250:
251:  # Front Jenkins with an HTTP server
252:  case node[:jenkins][:http_proxy][:variant]
253:  when "nginx"
254:    include_recipe "jenkins::proxy_nginx"
255:  when "apache2"
256>>   include_recipe "jenkins::proxy_apache2"
257:  end
258:


[2013-06-17T18:49:41+00:00] ERROR: Running exception handlers
[2013-06-17T18:49:41+00:00] FATAL: Saving node information to /var/chef/cache/failed-run-data.json
[2013-06-17T18:49:41+00:00] ERROR: Exception handlers complete
Chef Client failed. 0 resources updated
[2013-06-17T18:49:41+00:00] FATAL: Stacktrace dumped to /var/chef/cache/chef-stacktrace.out
[2013-06-17T18:49:41+00:00] FATAL: Chef::Exceptions::CookbookNotFound: Cookbook jenkins not found. If you're loading jenkins from another cookbook, make sure you configure the dependency in your metadata
```