Changing branch name to use for building staging site.

When the Jenkins roles for Ansible were first created, the `build_staging_site` job was configured to pull from the `working` branch of https://github.com/rackerlabs/developer.rackspace.com/ to build the staging site. 

After some discussion it was decided to call this branch `dev` instead of `working`. This PR is to make this change in the `build_staging_site` Jenkins job configuration file.