PresenceSuite.TestWatchPeriod reported nothing

[Curtis Hovey]

As seen at
    http://reports.vapour.ws/releases/issue/5668434d749a56277f023b2a

PresenceSuite.TestWatchPeriod has become more intermittent over the last 4 weeks.

presence_test.go:286:
    assertChange(c, ch, presence.Change{"a", true})
presence_test.go:77:
    c.Fatalf("watch reported nothing, want %v", want)
... Error: watch reported nothing, want {a true}

[Anastasia (anastasia-macmood)]

This failure looks to have been fixed as I cannot make this test to fail locally and under stress.