Update gorouter

$ git shortlog 25560809..acaa5860
Bleicke (1):
      Update readme [#45464553]

Kowshik Prakasam and Pieter Noordhuis (2):
      Check trace key before adding trace headers to response
      Refactor tests for trace key

Mike Heath (1):
      Added 'hosts' field to NATS 'router.start' messages so that DEAs can know the router's host fo

Pieter Noordhuis (2):
      Merge pull request #2 from mheath/master
      Add Travis widget to README

Change-Id: Iaa05a8eff2ad23c6a481c9ba49ad316aef663d5a