Include custom names in Jenkins job names

Previously, if you set up the same repo multiple times (e.g., to build
it on different platforms) like so:

    hubot ci setup github/janky janky18
    hubot ci setup github/janky janky19

...you'd end up with two nearly-indistinguishable Jenkins jobs with
names like:

    github-janky-2bce842af1a9
    github-janky-e89e28098c6a

With this change, the jobs will be easier to tell apart:

    janky18-2bce842af1a9
    janky19-e89e28098c6a

As an added bonus, the job names will usually be a bit shorter, giving
us still more breathing room on Windows before running into file path
length limits.
Merge pull request #158 from github/custom-job-names

Include custom names in Jenkins job names