Fix a test failure on 32-bit arches.

```
=== RUN   TestParseInvalidExpiry
    expiry_test.go:138: Invalid expiry '2147483647 days' did not have
    expected value (wanted: 2017-01-01 00:00:00 +0000 UTC, got:
    -5877594-06-22 00:00:00 +0000 UTC)
--- FAIL: TestParseInvalidExpiry (0.00s)
=== RUN   TestRevokeCmd
--- PASS: TestRevokeCmd (2.58s)
FAIL
FAIL	github.com/square/certstrap/cmd	2.592s
```