Fix display of inbound pushlog

https://bugzilla.mozilla.org/show_bug.cgi?id=1103440
I think you want to move this after the B2GNightly.__init__ call, else Nightly.__init__ will be called and will override your self.inbound_branch. Maybe something like:

B2GNightly.__init__(self, **kwargs)
if not self.inbound_branch:
    self.inbound_branch = B2GBuildsFinder.default_inbound_branch