Bug 1046743 - Make the job part number regex more generic

Prior to this patch each new job that became multi-part (eg xpcshell-1,
xpcshell-2, ...) would need adding to the regex. Since jobnames were
made more consistent, the generic regex doesn't hit false positives, so
we can use this instead for reduced maintenance.

Is the treeherder equivalent of:
https://hg.mozilla.org/webtools/tbpl/rev/b7a6c5411bd8