Allow updating existing job configurations

Quick synopsis... 

* We automate our deploys with Capistrano.
* Capistrano maintains a hash of Github repo to name.
* After a succesful deploy, the jobs are refreshed automatically based on this hash.  We essentially just whack  `localhost:5000/_hubot/setup?nwo=Owner/Repo&name=name` for each job.

Unfortunately, as it exists, Janky wants to create a new Jenkins job every time the corresponding `job.xml.erb` is modified and attached to a repository.  IMHO, the existing config should instead be updated using the Jenkins REST API.  The current setup leaves a big mess inside of Jenkins with extraneous jobs all over the place.

Fortunately the checks were already in place to see if a job exists, so I simply modified the code to update an existing job config if the job exists on the server already.

We're using this in production now.