make sure relative_time web helper is using a time object

The `relative_time` web helper assumes it will always be dealing with objects that respond to `.getutc`. Somewhere in our app, we're ending up with a `DateTime` object getting delivered here, which results in this exception:
```
NoMethodError - undefined method `getutc' for #<DateTime:0x000000018586c0>:
2014-03-21T20:20:02.206877+00:00 app[web.1]: /app/vendor/bundle/ruby/1.9.1/gems/sidekiq-2.17.7/lib/sidekiq/web_helpers.rb:93:in `relative_time'
```

We're not doing anything funky with `perform_async` or `perform_at` so I assume this is caused by some other 3rd-party gem like activesupport that affects the relationship of time-like objects. My change here seems like the safest way to guard against problems like this.