*Don't* silence the deprecation warnings in test

This is actually where we are most likely to see it (and we should try to deal
with those as soon as possible, ideally when upgrading).
FIX: honor `email_token_grace_period_hours` setting correctly

The documentation said that the unit for this setting is in *hours*, but the
code is treating it as *seconds*. Also, `Numeric#ago` has been deprecated since
Rails 4.1 (precisely to help prevent these kind of bugs) and will go away in
Rails 4.2.
Merge pull request #2868 from chancancode/dont_silence_deprecation_warnings

*Don't* silence the deprecation warnings in test