Sometimes GitHub doesn't return repo permissions

The Projects API isn't actually stable as of today, according to
https://developer.github.com/v3/projects/collaborators/. As such, no
assumptions can be made about what it returns. In older GitHub
Enterprise versions, for instance, the permission key is missing.

Change-Id: I3e706a209d263c698e76cb7e6b636d69d608ecc9

zuul/driver/github/githubconnection.py

@@ -1052,7 +1052,7 @@ class GithubConnection(BaseConnection):
             return 'none'
 
         # get permissions from the data
-        return perms.json()['permission']
+        return perms.json().get('permission', 'none')
 
     def commentPull(self, project, pr_number, message):
         github = self.getGithubClient(project)