Fix change number extraction on new enough Gerrit master

Apparently, there's been a change in recent upstream Gerrit which
changed the JSON serialization of change numbers in the output of
`gerrit stream-events`. Previously, the change number was being
transmitted as a string. That has changed and now it's being sent as a
number. That results in a failure later on:

 2017-02-14 15:35:55,294 ERROR zuul.IndependentPipelineManager: Exception while launching job XXX for change <Change 0x7f3efc185c90 64,1>:
  Traceback (most recent call last):
    File "/usr/lib/python2.7/site-packages/zuul/scheduler.py", line 1520, in _launchJobs
      dependent_items)
    File "/usr/lib/python2.7/site-packages/zuul/launcher/gearman.py", line 318, in launch
      destination_path = os.path.join(item.change.getBasePath(),
    File "/usr/lib/python2.7/site-packages/zuul/model.py", line 913, in getBasePath
      self.number[-2:], self.number, self.patchset)
  TypeError: 'int' object has no attribute '__getitem__'

I would love to have this extended this with a proper testcase. Please
feel free to push an updated version over this patchset, or just let me
know which place is the most appropriate and I can do it myself, too.

Change-Id: I68f5fa7fa4fac0b4d2eb232fe37df11e57157ecf

zuul/connection/gerrit.py

@@ -63,7 +63,7 @@ class GerritEventConnector(threading.Thread):
         if change:
             event.project_name = change.get('project')
             event.branch = change.get('branch')
-            event.change_number = change.get('number')
+            event.change_number = str(change.get('number'))
             event.change_url = change.get('url')
             patchset = data.get('patchSet')
             if patchset: