Removed `timecop` in favor of `travel_to`

With the release of `rails` 4.1, `Timecop.freeze` can be replaced with
`travel_to`

```ruby
Timecop.freeze 1.week.ago do
  # time sensitive code
end

travel_to 1.week.ago do
  # time sensitive code
end
```

Read more in the
[TimeHelpers](https://github.com/rails/rails/blob/a6f55fe257512731d7f3f41976648d99e9ec95be/activesupport/lib/active_support/testing/time_helpers.rb#L43)
module.