benchmark_test should validate against the same time delta regardless of when it's run

Previously used a constant timestamp 1384118307 and was thus failing the <20-days
test after twenty days had passed.

----------------------------------------------------------------------
FAIL: benchmark_test.go:366: IntegrationSuite.TestHttpPostWithTime

benchmark_test.go:384:
    c.Assert(data[0].Points, HasLen, 1)
... obtained [][]interface {} = [][]interface {}{}
... n int = 1

Received signal: terminated
OOPS: 6 passed, 2 skipped, 1 FAILED
--- FAIL: Test (35.65 seconds)