Race found in travis-ci

[Sean-Der]

Just from auto updating a lib #128

==================
WARNING: DATA RACE
Read at 0x00c000023f10 by goroutine 79:
  github.com/pion/sctp.TestRtxTimer.func4()
      /home/travis/gopath/src/github.com/pion/sctp/rtx_timer_test.go:206 +0x521
  testing.tRunner()
      /home/travis/.gimme/versions/go1.13.linux.amd64/src/testing/testing.go:909 +0x199
Previous write at 0x00c000023f10 by goroutine 78:
  github.com/pion/sctp.TestRtxTimer.func4.1()
      /home/travis/gopath/src/github.com/pion/sctp/rtx_timer_test.go:188 +0x89
  github.com/pion/sctp.(*testTimerObserver).onRetransmissionTimeout()
      /home/travis/gopath/src/github.com/pion/sctp/rtx_timer_test.go:95 +0x5a
  github.com/pion/sctp.(*rtxTimer).start.func1()
      /home/travis/gopath/src/github.com/pion/sctp/rtx_timer.go:155 +0x30d
Goroutine 79 (running) created at:
  testing.(*T).Run()
      /home/travis/.gimme/versions/go1.13.linux.amd64/src/testing/testing.go:960 +0x651
  github.com/pion/sctp.TestRtxTimer()
      /home/travis/gopath/src/github.com/pion/sctp/rtx_timer_test.go:183 +0xea
  testing.tRunner()
      /home/travis/.gimme/versions/go1.13.linux.amd64/src/testing/testing.go:909 +0x199
Goroutine 78 (running) created at:
  github.com/pion/sctp.(*rtxTimer).start()
      /home/travis/gopath/src/github.com/pion/sctp/rtx_timer.go:144 +0x1da
  github.com/pion/sctp.TestRtxTimer.func4()
      /home/travis/gopath/src/github.com/pion/sctp/rtx_timer_test.go:199 +0x38f
  testing.tRunner()
      /home/travis/.gimme/versions/go1.13.linux.amd64/src/testing/testing.go:909 +0x199
==================