Update use of deprecated Chef::ShellOut

This is what I get after each `build_jenkins_job` resource using Chef 11:

```
[2013-07-17T06:28:23+02:00] WARN: Chef::ShellOut is deprecated, please use Mixlib::ShellOut
[2013-07-17T06:28:23+02:00] WARN: Called from:
  /tmp/vagrant-chef-1/chef-solo-1/cookbooks/jenkins-job-builder/definitions/build_jenkins_job.rb:12:in `new'
  /tmp/vagrant-chef-1/chef-solo-1/cookbooks/jenkins-job-builder/definitions/build_jenkins_job.rb:12:in `block (3 levels) in from_file'
  /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.4/lib/chef/provider/ruby_block.rb:33:in `call'
  /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.4/lib/chef/provider/ruby_block.rb:33:in `block in action_run'
```

Perhaps we could check the chef version and run Mixlib::ShellOut otherwise.